Transistorised Differential Amplifier

Transistorised Differential Amplifier

The transistorised differential amplifier basically uses the emitter biased circuits which are identical in characteristics. Such two identical emitter biased circuits are shown in the Fig. 2.5.1.

The two transistors Q₁ and Q₂ have exactly matched characteristics. The two collector resistances R_C1 and R _C2  are equal while the two emitter resistances R_E1 and R_E2 are also equal.

Thus R _C1 = R_C2 and R_E1 = R_E2

The magnitudes of + V_CC and - V_EE are also same. The differential amplifier can be obtained by using such emitter biased circuits. This is achieved by connecting emitter E1 of Q₁ to the emitter E₂ of Q₂. Due to this, R_E appears in parallel with RE2 and the combination can be replaced by a single resistance denoted as RE. The base B₁ of Q₁ is connected to the input 1 which is V_S1 while the base B₂ of Q₂ is connected to the input 2 which is V_S2. The supply voltages are measured with respect to ground. The balanced output is taken between the collector C₁ of Q₁ and the collector C₂ of Q₂. Such an amplifier is called emitter coupled differential amplifier. The two collector resistances are same hence can be denoted asR_C.

The output can be taken between two collectors or in between one of the two collectors and the ground. When the output is taken between the two collectors, none of them is grounded then it is called balanced output, double ended outut or floating output.

When the output is taken between any of the collectors and the ground, it is called unbalanced output or single ended output.

The complete circuit diagram of such a basic dual input, balanced output differential amplifier is shown in the Fig. 2.5.2.

As the output is taken between two output terminals, none of them is grounded, it is called balanced output differential amplifier.

Let us study the circuit operation in the two modes namely

i) Differential mode operation and ii) Common mode operation.

1. Differential Mode Operation

In the differential mode, the two input signals are different from each other. Consider the two input signals which are same in magnitude but 180° out of phase. These signals, with opposite phase can be obtained from the center tap transformer. The circuit used in differential mode operation is shown in the Fig. 2.5.3.

Assume that the sine wave on the base of Q ₁ is positive going while on the base of Q₂  is negative going. With a positive going signal on the base of Q₁ an amplified negative going signal develops on the collector of Q₁ Due to positive going signal, current through R_E also increases and hence a positive going wave is developed across R_E. Due to negative going signal on the base of Q₂, an amplified positive going signal develops on the collector of Q₂. And a negative going signal develops across R_E, because of emitter follower action of Q₂. 

So signal voltages across RE due to the effect of Q1 and Q2 are equal in magnitude and 180° out of phase, due to matched pair of transistors. Hence these two signals cancel each other and there is no signal across the emitter resistance. Hence there is no a.c. signal current flowing through the emitter resistance. Hence RE in this case does not introduce negative feedback.

While V_o is the output taken across collector of Q₁ and collector of Q₂. The two outputs on collector 1 and 2 are equal in magnitude but

opposite in polarity. And V_o is the difference between these two signals, e.g. +10(-10) = + 20.

Key Point Hence the difference output V, is twice as large as the signal voltage from either collector to ground.

2. Common Mode Operation

In this mode, the signals applied to the base of Q₁ and Q₂ are derived from the same source. So the two signals are equal in magnitude as well as in phase. The circuit diagram is shown in the Fig. 2.5.4.

In phase signal voltages at the bases of Q₁ and Q₂ causes in phase signal voltages to appear across R_E, which add together. Hence R_E carries a signal current and provides a negative feedback. This feedback reduces the common mode gain of differential amplifier.

While the two signals causes in phase signal voltages of equal magnitude to appear across the two collectors of Q₁ and Q₂. Now the output voltage is the difference between the two collector voltages, which are equal and also same in phase, e.g. (10) - (10) = 0. Thus the difference output V_o is almost zero, negligibly small. Ideally it should be zero. 

Review Question

1. Explain the working of emitter coupled differential amplifier in differential mode and common mode.

May-05, 10, 18, Dec.-11, Marks 6