# bjt differential amplifier with constant current source

Note that it is customary to draw the base voltage line right through the transistor symbols for multiple current mirrors! Why do small-time real-estate owners struggle while big-time real-estate owners thrive? If Q4 was an ideal transistor, it would exhibit no internal losses and we would have a truly constant total emitter current regardless of the applied common mode voltage. Constant Current Source With Operational Amplifier: This instructable will show you how to make a current source with negligible current sag for loads with a resistance of less than 1.4k. This description is referenced to an input at the gate of NMOS1. A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.. A current source is the dual of a voltage source.The term current sink is sometimes used for sources fed from a negative voltage supply. p8.49 (simulate): design of BJT differential amplifier. ETIN70 –Modern Electronics: F9 –Differential and Multistage Amplifiers Differential and Common Mode Half Circuits • Differential mode: Push-pull anti-symmetry • Source output resistance: virtual ground •No differential current flow •Constant bias condition • Load resistance: split •Half the voltage level •Half the impedance value In this post, differential amplifier using BJT and differential amplifier using op-amps are explained in detail. The BJT Diﬀerential Ampliﬁer Basic Circuit Figure 1 shows the circuit diagram of a diﬀerential ampliﬁer. Would coating a space ship in liquid nitrogen mask its thermal signature? The object is to solve for the small-signal output voltages and output resistances. READ PAPER. Smallest known counterexamples to Hedetniemi’s conjecture. amplifier is in principle: A two-stage amplifier consisting of a common-gate stage (NMOS 2) driven by a common-drain stage (NMOS1). We know that in a BJT the current flowing through the base of the transistor controls how much transistor is closed, ... You can also check the constant current circuit which can also be used to drive current-controlled devices. Resistance has a voltage-current relationship as per the ohms law. Therefore, when the input differential voltage v D = v B2 – v B1 changes in time, some of the current of a given transistor will be transferred to the other. BJT one stage amplifier with current source, Podcast 305: What does it mean to be a “senior” software engineer. $$I_C = \frac{4.5\textrm{V}}{1\textrm{k}\Omega} = 4.5\textrm{mA}$$. V. T (3) The difference input signal, v. id. Multiple current mirrors may be slaved from a single (Q1 - Rbias) voltage source. The BJT differential AMP with an active load • Many IC amplifiers use BJT loads in place of the load resistance, R C. • BJT load resistor is usually connected as a constant-current source with a very high resistance load (output resistance of the current source) • Higher load resistance, higher output gain. Tail Resistance. shows that, the offset is higher than that of a resistive loaded differential amplifier A. Why did the design of the Boeing 247's cockpit windows change for some models? Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest? The higher the resistance of the current source R tail, the lower the common mode gain or A c is, and the better the common mode rejection ratio (CMRR). Why? As shown again in figure 4, the BJT operates in the cutoff region when I B is equal to zero. Assuming the three tarnsistors are matched with Vsegi =Vseq2 =Vsegs =+0.7V&B B92 =B03=120.If the input AC voltages Vin] =-2.5mA & Vin2=28mA a) Calculate the DC emitter-current of Q3 b) Calculate the DC base-currents of Q1 & Q2 c) Calculate the differential-mode gain Ay(dm) d) … Therefore, we need to determine the common mode gain of … What is the "Ultimate Book of The Master". For CE amplifier \$R_L\$ >> \$R_C\$ Do you know why? 7. Also, you have error in your circuit. BJT_CURRENT_SOURCE.CIR Download the SPICE file. By adding \$C_E\$ capacitor we are bypassing (shunt) this large output resistance by a very small \$X_C\$ capacitive reactance. The current transfer characteristic curve showing the plot of collector current of transitor Q 1 and Q 2 versus the differential input voltage V in(d) is shown in Fig. The amplifier has several variations on this basic configuration. Therefore, if we have a w… In order to implement a successful current mirror, one transistor (here, ) must have a current induced in it to mirror it to the differential amplifier’s current source (here, ). Differential amplifiers can be made using one opamp or two opamps. $$A_v = - \frac{R_C||R_L||r_{o1}}{r_{e1}} \cdot \frac{\beta}{\beta+1}$$. • Design a diff-amp with an active lo a dt y iesp c f differential-mode voltage gain. Differential Amplifier with Active Loads Active load basics PNP BJT current mirror ... parallelcurrent sources. Published under the terms and conditions of the, Introduction to Bipolar Junction Transistors (BJT), The Bipolar Junction Transistor (BJT) as a Switch, Active Loads in Amplifier Circuits Worksheet, Differential Transistor Amplifiers Worksheet, How EE Lab Education is Evolving with At-Home Learning, An LCD for Your Microcontroller Using a Single I/O, Utilizing Open Source Hardware in Academic Environments, Current and Voltage Relationships in Bipolar Junction Transistors (BJTs). The load resistors are drawn almost invisible to emphasize the fact that these do not exist in most cases. A differential amplifier multiplies the voltage difference between two inputs (Vin+ - Vin-) by some constant factor Ad, the differential gain. The tail supply is modeled as a current source IQ. For an example of a current mirror with multiple collector, outputs see Q13 in the model 741 op-amp, Ch 8. p8.34: input common mode range of BJT differential amplifier. Question: My objective is to understand the need of the tail current source in a differential amplifier.. Answer: Consider, what the diff. The Collector -emitter voltages of Q 1 and Q 2 are given by. by Neil Zhao, Reem Malik, and Wenshuai Liao Download PDF Precision current sources provide a constant current in many applications, including industrial process control, instrumentation, medical equipment, and consumer products. CH 10 Differential Amplifiers 16 Differential Response II X CC Y CC C EE C C EE V V V V R I I I I 1 0 2. V CE1-V CE2 =V C-V E =V CC - V EB-(-V EB)= V CC. Create one now. We see from these examples that current mirrors are preferred as loads over resistors in integrated circuitry. Which is warmer for slipper socks—wool or acrylic? For example, in figure 6 we have a BJT circuit which is similar to the circuit that we’ve analyzed recently though their difference … Please go through both of them to get a better understanding. Bipolar Junction Transistors: Current Mirrors 2. Eqn. Is the current gain value constant for a single BJT ? Besides supplying the constant emitter current, constant current bias provides a very high source resistance also since the AC equivalent or DC source is ideally an open circuit. This description is referenced to an input at the gate of NMOS1. The BJT operates in the saturation region when its collector current is not dependent on the base current and has reached a maximum. The amplifier is to have a differential gain (to each of the two outputs) of at least 100 V/V, a differential input resistance ≥10k Ω and a common mode gain (to each of the two outputs) no greater than 0.1 V/V. The total emitter current is kept constant by the current source I. Often, the collector load of a transistor is not a resistor but a current mirror. Discrete Semiconductor Circuits: Current Mirror 3. How can I use Mathematica to solve a complex truth-teller/liar logic problem? Applications of Current Mirror Circuit. It will be assumed that the transistors are identical. Common emitter bjt amp Q point with and without current mirror. Thus, all the performance equations obtained for the differential amplifier by … The condition for this to happen is that both the base-emitter and the base-collector junctions should be forward-biased. 1) MOS and the bipolar differential amplifiers: how they reject common-mode noise or interference and amplify differential signals 2) The analysis and design of MOS and BJT differential amplifiers: utilizing passive resistive loads, current-source loads, and cascodes 3) The structure, analysis, and design of amplifiers A short summary of this paper. Differential Amplifier with Active Load Reference: Neamen, Chapter 11 (7) Learning Outcome Able to: • Describe active loads. p8.62 (simulation only): NPN differential amplifier fast switching (ECL logic) by current steering: e.g. The device has two differential amplifier units (10,20), two level shifters (30,40), a current switch (50) between the amplifier units for dividing the common mode input region associated with them and a first constant current source (60). For example the collector load of Q4 collector, Ch 8 is a current mirror (Q2). – The input impedance is obtained by applying a small change in the … Therefore \$Q_2\$ base voltage needs to be around \$1.7\textrm{V}\$. This change in transistor current … $$R_2 =\frac{V_B}{5\cdot I_B} =\frac{1.7V}{5\cdot \frac{4.5\textrm{mA}}{100} }= 7.5\textrm{k}\Omega$$, $$R_1 =\frac{V_{CC} - V_B}{6\cdot I_B} =\frac{8.3V}{6\cdot \frac{4.5\textrm{mA}}{100} }= 30\textrm{k}\Omega$$. Q2 cannot be saturated? The quiescent current has to be constant to ensure constant collector voltages at common mode. I Assumed 1V of a headroom. Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. Current sources are needed in a number of different areas of electronics circuit design. CH 10 Differential Amplifiers 15 Differential Response I Y CC X CC C EE C C EE V V V V R I I I I 2 0 1. Or in the case of Q4 in the figure above, two current sources are associated with a single transistor symbol. Which is not the case here. This is because they are used 1) for biasing and 2) as active loads. Thanks for contributing an answer to Electrical Engineering Stack Exchange! The BJT current source (top of page) produces a constant current out of Q4 regardless of the voltage at it's collector. How to find values of that resistors if unpredictable resistance of current source exists in emitter of Q1. Analysis of BJT differential amplifier with active load: The collector currents of all the transistors are equal. Assuming the three tarnsistors are matched with Preps =Vseg: Pegs +0.7V&Bo Bez Bo=220.If the input AC voltages Vin=2.5mA & Vn2=28mA a) Calculate the DC emitter-current of Q3 b) Calculate the DC base-currents of Q1 & Q2 c) Calculate the differential-mode gain Adm) d) Calculate the common … BJT differential amp with current mirror biasing Differential amplifiers have high common mode rejection ratio (CMRR) and high input impedance. range of BJT diff pair is smaller than the MOS diff pair (2) It can be used for . The CE stage gain is \$\Large\frac{R_C||R_L}{r_{e1}+r_{o2}}\$ where \$r_{o2}\$ is \$Q_2\$ output "resistance" seen from \$Q_2\$ collector into \$Q_2\$. Differential amplifier unit output currents are cross-connected to form additional output currents. Figure 3: An improved long-tailed pair with current-mirror load and constant-current biasing. vB1 = vB2 = vcm: IQ splits equally … In circuit lab the DC analyzer show that Q1 emitter voltage is 222 nV. Insulated-Gate Field-Effect Transistors (MOSFET) I see that advantage that see all textbooks authors - now I have constant Q1 CE current which makes Q-point stable and hard-fixed on 1/2 of Vcc or it's not so? It plays two positions on the audio team: 1) bias the differential input stage and 2) provide bias AND a super high gain for the Gain Stage with its astonishingly high output resistance. It may have either one output or a pair of outputs where the signal of interest is the voltage difference between the two outputs. The small signal model greatly simplifies an audio amp into three stages: a differential amplifier (GDIFF), a voltage gain stage (Q3) and an output buffer (EBUFF). Question-2 BJT based differential amplifier with a constant curent-source. The reference current source is created using the current mirror circuit. Constant Current Source With Operational Amplifier: This instructable will show you how to make a current source with negligible current sag for loads with a resistance of less than 1.4k. To get the maximum voltage swing I decided to pick the \$V_{Rc}\$ value as: $$V_{Rc} = \frac{(V_{CC} - V_E)}{2} = \frac{10\textrm{V} - 1\textrm{V}}{2} = 4.5\textrm{V}$$, So, if \$R_C = 1\textrm{k}\Omega\$ we have : unless you’re an IC designer. So let’s start with the cutoff region as it is the easiest one to understand. Use MathJax to format equations. • Active loads are essentially transistor current sources used in place of the resistive loads in the diff-amp circuits to increase differential-mode gain. An often-used circuit applying the bipolar junction transistor is the so-called current mirror, which serves as a simple current regulator, supplying nearly constant current to a load over a wide range of load resistances.. We know that in a transistor operating in its active mode, the collector current is equal to base current … amplifier is in principle: A two-stage amplifier consisting of a common-gate stage (NMOS 2) driven by a common-drain stage (NMOS1). The long tailed pair or differential pair is extensively used in integrated circuit technology, especially in operational amplifiers where it provides the basic building block of the whole amplifier. Though rarely encountered in typical PCB design, current sources are ubiquitous in the world of analog ICs. Why are both bias resistors considered parallel in a common emitter amplifier? BJT Differential amplifier. This paper. The\$r_{o2}\$ "resistance" is large so, the stage voltage gain will be very small. Thus, all the performance equations obtained for the differential amplifier by using the emitter bias … Constant-current sources figure prominently in circuit-analysis exercises and network theorems, then they seem to more or less disappear . The differential amplifier can be implemented with BJTs or MOSFETs. The basic configuration (Figure 1-1) will be studied in this experiment. Input 1 is a 40Hz signal, and input 2 is a 40Hz signal with some small voltage spikes added in. BJT Amplifiers: Overview. Current sources can be used to bias transistors and can also be used as active loads for high gain amplifier stages. 8/27/2018 5 Voltage Amplifier • In an ideal voltage amplifier, the input impedance is ... – All id dindependent sources are set to zero. Better yet fabricate one transistor, say Q3 with twice the area of Q2. Differential amplifier is a closed loop amplifier circuit which amplifies the difference between two signals. Question: My objective is to understand the need of the tail current source in a differential amplifier.. Answer: Consider, what the diff. Do you see any advantage of this topology? Unfortunately, this is not the case. This type … Bipolar Junction Transistor or BJT Current Mirror. Transistor long tailed pair with a current mirror . That is why, in more sophisticated designs, an element with high differential (dynamic) resistance approximating a constant current source/sink is substituted for the “long tail” (Figure 3). And now you can play with this circuit in your circuit simulator program. • BJTs {MOSFETs} … In this project, we will explain how a voltage-controlled current source using op-amp can be designed and also build it to demonstrate its working. I C1 = I C2 = I C3 =I C4 = I EE /2 . ex8.13: BJT diff pair: differential gain, CMRR. R_L >> R_C because of load resistance must not to overload an amplifier, another way the Av grain of CE stage will be very small. I can't imagine how a current source on the emitter would help anything. Weve seen already how maintaining a constant base current through an active transistor results in the regulation of collector current, according to the β ratio. Feedback resistors R2 and R1 close the loop on the amplifiers for a gain of 10x. One of the less glamorous but equally important circuits in audio amplifiers is the current source. Now we can choose the voltage divider resistors values. In order to implement a successful current mirror, one transistor (here, ) must have a current induced in it to mirror it to the differential amplifier’s current source (here, ). Input Resistance, Understanding constant current source in amplifier stage. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. The quiescent current has to be constant to ensure constant collector voltages at common mode. Why is “HADAT” the solution to the crossword clue "went after"? The Q13 current mirror outputs substitute for resistors as collector loads for Q15 and Q17. 2. 11.4. Iload, parallel Q2 and Q3. The constant current source (3) output (19) can be connected to one input of the first differential amplifier (5) via a switch (21) for changing between 3 and 4 conductor circuit resistance measurement. Op-Amp based Constant Current Source: We know that, the circuit which provides constant current to the load (irrespective of change in load resistance) is called as constant current source circuit. • Design a diff-amp with an active lo a dt y iesp c f differential-mode voltage gain. • Active loads are essentially transistor current sources used in place of the resistive loads in the diff-amp circuits to In more sophisticated designs, an active constant current source may be substituted for the high resistance R tail. Due to the fixed tail current source, the input common-mode value can vary without changing the output common-mode value. Differential Amplifier with Active Load Reference: Neamen, Chapter 11 (7) Learning Outcome Able to: • Describe active loads. Q2 1V headroom is fixed with R1/R2 divider in your circuit? The BJT Differential Amplifier. How I can correct my schematic to properly set the current source in the emitter of BJT common emitter stage? Figure 1-1 Basic BJT differential … Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Does it take one hour to board a bullet train in China, and if so, why? This requirement is not so important in the case of a differential output since the two collector voltages will vary simultaneously but their difference … The classic differential pair amplifier is formed from at least two identical transistors, configured with the emitters for BJT transistors or the sources for FETs connected together. This change in transistor current with input differential variation can be observed in fig. The typical BJT differential pair amplifier consists of a pair of transistors coupled at the emitters to a current source, having equal resistances in each collector and equal but opposite, signal sources in each base. A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.. A current source is the dual of a voltage source.The term current sink is sometimes used for sources fed from a negative voltage supply. BJT_CURRENT_SOURCE.CIR Download the SPICE file. Well, the α ratio works similarly: if emitter current is held constant, collector current will remain at a stable, regulated value so long as the transistor has enough collector-to-emitter voltage drop to maintain it in its active mode. How can you … Without this additional capacitor, the voltage gain is very low. 27 Full PDFs related to this paper. It is usually implemented by a current mirror because of its high compliance voltage (small voltage drop across the output transistor). The corrected circuits diagram looks like this, simulate this circuit – Schematic created using CircuitLab. A current mirror will sink a more or less constant current over a wide range of voltages. Hence, changing one reference point also change the current source across … Download Full PDF Package. Therefore, when the input differential voltage v D = v B2 – v B1 changes in time, some of the current of a given transistor will be transferred to the other. ETIN70 –Modern Electronics: F9 –Differential and Multistage Amplifiers Differential and Common Mode Half Circuits • Differential mode: Push-pull anti-symmetry • Source output resistance: virtual ground •No differential current flow •Constant bias condition • Load resistance: split •Half the voltage level •Half the impedance value In view of the ease of use of operational amplifiers and their low cost, long tailed pair circuits made from discrete components are seldom seen. bjt_diffamp1.cir - bjt differential amplifier * * signal source vs 1 2 ac 1 sin(0 10mvpeak 10khz) vcm 2 0 sin(0 0mvpeak 5khz) * * power supplies vcc 11 0 dc +15v vdd 12 0 dc -15v * q1 3 1 5 q2n2222 q2 4 2 5 q2n2222 rc1 11 3 1000 rc2 11 4 1000 re 5 12 7.2k * * .model q2n2222 npn(is=3.108f xti=3 eg=1.11 vaf=131.5 bf=217.5 ne=1.541 + ise=190.7f ikf=1.296 xtb=1.5 … The amplifier is to have a differential gain (to each of the two outputs) of at least 100 V/V, a differential input resistance ≥10k Ω and a common mode gain (to each of the two outputs) no greater than 0.1 V/V. Biasing: Transistors … The nonconducting state of the BJT falls under the cutoff region. Will SQL Server go offline if it loses network connectivity to SAN where master and msdb system databases reside? should be less than . Differential amplifier. The load is often another (multiple) transistor circuit, say a pair of emitters of a differential amplifier, for example, Q3 and Q4 in “A simple operational amplifier”,Ch 8. The constant-current source circuit includes one or more constant-current source circuit … Thus, AC output resistance plays a major role in the stability … Besides supplying the constant emitter current, constant current bias provides a very high source resistance also since the AC equivalent or DC source is ideally an open circuit. Therefore, for an ideal current amplifier, the current transfer ratio is an important parameter. CH 10 Differential Amplifiers 17 Differential … 4. Figure 1: Circuit diagram of the diﬀerential ampliﬁer. This indicates the success of the op amp … i c2≈g mn v dm 2 =i c4 v o−dm= i c2 i c4 R o=2 g mn v dm 2 R o v o−dm=2g mn R o v dm 2 G dm= v o−dm v dm =g mn r op∥r on vdm/2 vdm/2 Current mirror circuits may be found in two basic varieties: the current. An inverting differential amplifier has inputs connected to the return supply line and a null line and its output connected to one of two measurement lines. The BJT differential AMP with an active load • Many IC amplifiers use BJT loads in place of the load resistance, R C. • BJT load resistor is usually connected as a constant-current source with a very high resistance load (output resistance of the current source) • … But the AC gain for this circuit arrangement will be one or even less than one. Q3 is the control transistor for the current source Q4. Download PDF. Download. Since \$I_C = 4.5\textrm{mA}\$ and the \$\beta_{min} = 100\$ I decide to pick the resistor values this way. Also, as you can see I add \$C_E\$ capacitor to increase the gain. $$R_3 = \frac{10\textrm{V} - 0.7\textrm{V}}{4.5\textrm{mA}}= 2\textrm{k}\Omega$$. How to find values of that resistors if unpredictable resistance of current source exists in emitter of Q1. Left figure shows an AMP with active load (consisting of Q3 … The advantage of a curre… However, the role of both … Design a BJT differential amplifier that provides two single-ended outputs (at the collectors). This comes in use when a circuit needs a steady current supply, without fluctuations. If a jet engine is bolted to the equator, does the Earth speed up? The voltage divider current should be at least ten times larger the base current. V. T /2 to linear amplification. The temperature compensation of the output current of the constant-current source circuit is effected by using the threshold voltage characteristic of a transistor such as a MOS transistor or a bipolar transistor. How I can correct my schematic to properly set the current source in the emitter of BJT common emitter stage? The BJT Diﬀerential Ampliﬁer Basic Circuit Figure 1 shows the circuit diagram of a diﬀerential ampliﬁer. BJT Diff Pair. current flows entirely in one branch then switches to the other branch; requires only 4. The amplifier has several variations on this basic configuration. A constant-current source circuit and a differential amplifier using the same. ... BJT Differential Amplifier – Circuit and explanation; And \$R_1\$ and \$R_2\$ voltage divider do just that. The typical BJT differential pair amplifier consists of a pair of transistors coupled at the emitters to a current source, having equal resistances in each collector and equal but opposite, signal sources in each base. x x i v impedance 8/27/2018 6 Input Impedance Example • Note that input/output impedances are usually regarded as small‐signal quantities. In other words, load current scales with the transistor area. Emitter constant current source. does paying down principal change monthly payments? Transistors in a current mirror circuit must be maintained at the same temperature for precise operation. 1. By using this technique, multiple reference points can be created from a single source. When using discrete transistors, you may glue their cases together to do this. Figure 11.4: The current transfer characteristic curve of a bipolar junction transistor differential amplifier From the characteristic curve, once can notice that for several V T values such as Vin(d) > 4V T, either I … The output is equal to the voltage difference between the two inputs. Q4 is open collector and so exhibits a rela-tively high output impedance, that is, it approximates a constant current source. It will be assumed that the transistors are identical. I think that it must be so because of saturation case the mirrored current will be very big. Use a 2mA current source for biasing. Should I hold back some ideas for after my PhD? In our example using a power supply, the current provided is 9.1 mA but can be adjusted.