Transfer Characteristics of Differential Amplifier

In this lecture, we will discuss the next important topic of differential amplifiers which is the Transfer Characteristics of Differential Amplifiers. We will see the transfer characteristics in detail

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Also Read: Ideal Differential Amplifier

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Transfer Characteristics of Differential Amplifier

The DC Transfer Characteristics of Differential Amplifier is useful in understanding the large signal behavior of the differential amplifier. This is important because the transfer characteristics show the circuit operation is linear over a limited range of input voltage.

Assumption Made for Transfer Characteristics

The assumptions made in plotting the transfer characteristics of the differential amplifier are as follows:

1. The resistance of the current source in the above figure is infinite.
2. The base resistance of the transistors is neglected.
3. The output resistance of each transistor is infinite.
4. The differential input voltage V_d = ( V_B1 – V_B2).

Description of Transfer Characteristics of Differential Amplifier

The Transfer Characteristics of the Differential Amplifier is shown in the above figure. It is the graph of normalized differential input voltage versus the normalized collector current. The normalized differential input voltage is defined as the ratio \mathbf{\left[\frac{V_d}{V_T} \right ]}.

For the negative values of normalized differential input voltages, (V_B1 – V_B2) is negative. That means V_B1 is smaller than V_B2. Therefore transistor Q₂ will conduct more than transistor Q₁. For normalized differential input voltage higher than -4V, Q₁ is OFF and Q₂ only is ON> Therefore only I_C2 will flow.

Then for the differential input between -4V and 0V, both the transistors conduct with I_C2 > I_C1. And at 0 differential input voltage both the transistors conduct equally, i.e. I_C1 = I_C2.

With the increase in the normalized differential input voltage in the positive direction, Q₁ starts conducting more than Q₂ goes towards cutoff.

The linear region is confined to the zero normalized differential input voltage as shown in the above figure.

Relation between output voltage and differential input voltage of Differential Amplifier

The output voltage is the difference between the collector voltages of the two transistors. The variation of output voltage with respect to the input voltage is shown in the below figure.

The advantages of the differential amplifier are that for zero differential input voltage, the output voltage also is equal to zero.

Conclusions

1. The linear range of operation is limited to a very small value about the zero differential voltage.
2. The state of output voltage changes between two distinct values if the differential input voltage is large enough. This will allow us to the differential amplifier as a switch in the applications such as ECL gates (Emitter coupled logic gates).

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Also Read: Differential Amplifier With Constant Current Source

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