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Long-tailed Pair -- Revision of Results Differential-mode: Av = (-)0.5Rchfe/hie Rin = hie at each input Common-mode: Av = -Rchfe/[hie + 2(1 + hfe)Re] The Current Mirror VCC R i v 0V The Current Mirror is a circuit which can provide an almost constant direct current (which means it has a very high effective internal resistance, so its current is very little affected by the load we connect to it). If the transistors are identical (same dimensions, adjacent on the same chip) and they have the same VBE, the only difference in their collector currents will be as a result of different VCE values. As IC mostly depends on IB (and therefore VBE) rather than VCE, the collector current of the right-hand circuit largely depends on that of the left-hand circuit which we can set by choosing a suitable value of Rc. (IC is approximately (VCC - 0 - VBE)/R). In order to see how constant IC is as we vary VCE for the right-hand transistor, we need to find out the relation between the signal values of voltage and current v and i. So we draw the equivalent circuit: VCC (= 0 V for signals) 1/hoe R ib2 X hfeib1 i hfeib2 hie hieie1 1 1/hoe v ib1 0V This looks like another case of “Danger! Hard Sums” but for once all we need is Ohm’s Law and common sense. A little thought reveals that no current can be flowing in anything but hoe for Transistor 2. The argument is as follows. The top of resistance R is at 0 V for signals .. so the current in R is equal to the voltage at X divided by R and it will flow upwards if the voltage at X is positive. The bottom of both hie, and hoe for Transistor 1, is also at 0 V, so the current in all of those resistances will flow downwards if the voltage at X is positive; the same will then apply to the generator currents. All the current will therefore be flowing out of Point X, which it cannot by Kirchhoff’s Current Law. If the voltage at X were negative, all the current would flow into X, which it is again not allowed to do by Kirchhoff’s Current Law. Therefore the voltage at X must be zero and no current flows in either generator. The current I can now only flow through hoe for Transistor 2, so the signal resistance faced by the current I will be 1/ hoe. This signal resistance will typically be tens of kilohms -- a lot higher than we could achieve by just using a resistor. We can obtain an estimate of hoe, by the way, from the transistor data even if the manufacturer does not give full output characteristics if the Early voltage is quoted (which it usually is). The beginner often ignores this piece of data on the basis that it will probably have disappeared before the alarm clock goes off. In fact it is a very useful piece of data, as the following diagram shows: IC - Early Voltage VCE So if we know the Early Voltage (often referred to as VA) and the quiescent values of IC and VCE, we can estimate hoe as IC/(VCE + |VA|). An example to show the improvement 12 V 4.7 k 4.7 k vo vi1 With an actual resistor for Re vi2 4.7 k -12 V We remember (or have forgotten) that the current in the emitter resistor can be calculated as [0 - VBE - (-12)]/Re = 11.4/4.7 = 2.43 mA. We now require to produce a Current Mirror which will give the same current. The design calculation, for once, is a simple d.c. one. In the circuit on the next page, we have a current path through R and a base-emitter junction from +12 V to -12 V. The current in R will therefore be [12 - (12) -0.6]/R, and we want its value to be 2.43 mA. We therefore solve: [12 - (12) -0.6]/R = 2.43 (with a TI - 85 if we wish!) and conclude that R must be [12 - (12) -0.6]/2.43 = 9.65 k. We would manufacture to the exact value if we were making the circuit in integrated form, but probably use a 10-k resistor if it were to be manufactured from discrete components. 12 V 4.7 k 4.7 k vo vi1 R vi2 With a Current Mirror for Re -12 V Level Shifting The output voltage of the long-tailed pair circuits above will not be zero when both the inputs are zero. As the circuits stand, the quiescent VO will be 12 - (4.7 2.43/2) = 6.3 V; it would be much better if it were zero! Several methods exist of making the quiescent value zero. 1. Take the output via a capacitor. This is a good solution for an a.c. amplifier, but it will not work for d.c. or indeed slow a.c. Anyone who has tried to measure slow signals on an oscilloscope set to “a.c.coupled” knows this only too well! 2. Use a voltage divider. This will work up to a point, but it will have two problems: it will divide the signal as well (losing some of our hard-earned amplification!) and it will either load the output significantly or give rise to a high output resistance. 3. (A variant of 2). Use an emitter (or source) follower. This will largely overcome the problems of (2) but will produce only a slight shifting of the level. 4. Use an emitter (or source) follower in conjunction with a current mirror. This often proves to be the best method in practice. Suppose we attach the circuit on the next page to the output of our long-tailed pair. The long-tailed pair output “sits” at 6.3 V, so the emitter of the emitter-follower transistor will be at 6.3 - 0.7 = 5.7 V. We now want to drop that 5.7 V across the 4.7k resistor. (4.7 k was chosen as a “reasonable value” - it need not be 4.7 k). The current in the current mirror will therefore need to be 5.7/4.7 = 1.21 mA. The resistance R must therefore carry 1.21 mA, so R = [12 - 0.6 - (-12)]/1.21 = 19.3 k. Again we could use the precise value in integrated form, but in discrete form we would be well advised to use the next lower preferred value of 18 k, and to incorporate a low-value potentiometer in series with the 4.7 k resistor to adjust the quiescent output voltage to zero. The base current of the emitter-follower transistor will only be 1.21/hFE mA, so it is unlikely significantly to affect the collector voltage or current of the right-hand transistor in the long-tailed pair. 12 V From L.T.P. output 4.7 k R vo -12 V Some Questions 1. If the Early voltage of its transistors is 25 V, what will be the signal resistance of the left-hand current-mirror transistor? (30.6 k) 2. How much voltage gain will be lost as a result of the level shifting? (Reduced to 30.6/(30.6 + 4.7) = 0.867 of its value) 3. Could we use a similar circuit instead of Rc to increase the voltage gain of the circuit? (Yes). 4. A transistor differential amplifier (“Long-tailed pair”) has both collector resistors of value 3.9 k and the emitter resistor of value 3.3 k. The transistors are identical with hfe = 300, hie = 5 k, and negligible hoe and hre. The quiescent base-emitter voltage drop is 0.6 V. a) If it operates on 12 V supplies, what will be the quiescent collector current and voltage? (1.73 mA, 5.26 V) b) Determine the differential- and common-mode voltage gains and the commonmode rejection ratio. (-0.71, -282 (-141 based on difference of inputs)) c) Explain how: (i) the CMRR could be improved by a suitable circuit modification. (ii) the voltage gain could be improved by a suitable circuit modification. (iii) the output voltage could be made 0 V for 0 V input. d) What will be the signal output voltage of the circuit if 10 mV is applied to the base of the transistor from whose collector the output voltage is taken and 0 V is applied to the base of the other transistor? (-1.41 V)