FET Differential Amplifier

FET DIFFERENTIAL AMPLIFIER

The JFET differential amplifier with resistive load is considered for the analysis. MOSFETs are widely used in the input stages due to their better stability.

i. FETs are used instead of BJT when we require

a. High Impedance

b. Better Stability

ii. Fig.3.23 shows the dual input balanced output differential amplifier using JFET.

Apply KVL to the input loop,

We know that

The differential voltage in the input

V_ID = V_I1 - V_I2

Assuming Q₁ and Q₂ as ideal transistors, then

V_P1 = V_P2

I_SS = I_D1 + I_D2

JFET differential amplifier can handle larger input signals than BJT differential amplifier. The drain output voltage is obtained from the Fig.3.23 is

Then the output differential voltage

For smaller differential input voltage, the transfer function is approximately linear. The transfer characteristics of FET differential amplifier is shown in Fig. 3.24.

The differential voltage gain is the ratio of differential output voltage to differential input voltage

This gives the expression for voltage gain of FET differential amplifier.

MOSFET DIFFERENTIAL PAIR

Fig. 3.25 shows the basic MOSFET differential pair. MOSFETs M₁ and M₂ are matched to each other in all respects.

i. I_Q is a constant current source similar to that of BJT differential amplifier.

ii. Both the MOSFETs are biased in their saturation regions.

iii. Dual polarity dc supply is used. So, even if V_GI = V_G2 = 0, both the MOSFETs are operated in the saturation region.

Features of FET Differential Amplifiers

i. Very high input impedance

ii. Low differential gain

iii. Low input bias and offset current

iv. More noise.