JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ELECTRONIC CIRCUIT ANALYSIS Previous Question Paper (July/August - 2021 - R15)

A Common Emitter (CE) amplifier using voltage divider bias consists of a transistor with resistors R1 and R2 forming a voltage divider at the base, an emitter resistor RE bypassed by a capacitor CE, and a collector resistor RC. The input signal is coupled via capacitor CC1 to the base, and the output is taken from the collector via CC2.

To obtain the AC equivalent circuit, all DC sources are shorted to ground and all coupling/bypass capacitors are treated as short circuits. In the hybrid-pi model, the transistor is replaced by a controlled current source g_mV_be (or βI_b) in parallel with output resistance r_o, and an input resistance h_ie (or r_π) between base and emitter.

The expressions for the performance parameters are derived as follows:

1. Input Impedance (Z_i): The input impedance is the parallel combination of the biasing resistors and the transistor input resistance. Z_i = R1 || R2 || h_ie.
2. Output Impedance (Z_o): Looking into the collector terminal with the input source shorted, the output impedance is Z_o ≈ RC (assuming r_o is much larger than RC).
3. Voltage Gain (A_v): The voltage gain is the ratio of output voltage to input voltage. A_v = -h_fe(RC || R_L) / h_ie. The negative sign indicates a 180-degree phase shift.
4. Current Gain (A_I): The current gain is defined as the ratio of load current to input current. A_I = -h_fe(R1 || R2) / (R1 || R2 + h_ie).

A cascode amplifier consists of a common-emitter (CE) stage followed by a common-base (CB) stage. This configuration provides high voltage gain, high input impedance, and excellent isolation between input and output, minimizing the Miller effect.

The AC equivalent circuit consists of transistor Q1 in CE configuration with input signal applied to its base, and transistor Q2 in CB configuration acting as the load for Q1. The small-signal model replaces Q1 and Q2 with their hybrid-π models, where g_m is transconductance and r_o is output resistance.

1. Input Resistance (R_in): Since the input is applied to the base of Q1, the input resistance is simply the base resistance of the first stage:

2. Output Resistance (R_out): The output is taken at the collector of Q2. The CB stage (Q2) provides a high output impedance. Including the finite output resistance of the transistors, the output resistance is approximately:

3. Voltage Gain (A_v): The voltage gain is the product of the gain of the CE stage and the CB stage. The CE stage acts as a transconductance amplifier, and the CB stage acts as a current buffer. The overall gain is:

In the ideal case where internal resistances are large, this simplifies to A_v ≈ -g_m1g_m2r_o1r_o2, demonstrating the high gain capability of the cascode structure.