Gain and Frequency Response of High Frequency Analysis

GAIN AND FREQUENCY RESPONSE OF HIGH FREQUENCY ANALYSIS

GAIN AND FREQUENCY RESPONSE OF BJT AMPLIFIER

Fig.2.28 shows the high frequency equivalent circuit of CE amplifier. All the coupling and bypass capacitors have been assumed to be short circuited and wiring capacitance is ignored.

The Fig. 2.29, can be simplified using Millers theorem as follows.

The input and output feedback capacitance C_b'c are replaced by the input and output shunting capacitances. Fig. 2.29 shows the modified circuit of CE amplifier.

Let us find the Thevenin's equivalent circuit of the input circuit.

The Thevenin's imopedance R_th is the impedance looking from B and E. To calculate R_th asume V_S to be short circuited and C_eq is open circuited. Then Thevenin's voltage is measured as the open circuit voltage measured across B and E.

The Thevenin's voltage is obtained as

The simplified Thevenin's circuit is shown Fig.2.30.

From the Thevenin's equivalent circuit,

where  = upper 3dB cutoff frequency of input circuit

Consider the output circuit,

Where  = upper 3 dB cutoff frequency of output circuit

We know that voltage gain, 

Substitute (1) and (2)

Hence A_V can be written as

This expression gives the voltage gain of the BJT amplifier.

GAIN AND FREQUENCY RESPONSE OF FET AMPLIFIER

The equivalent circuit of FET amplifier is shown in Fig.2.31. The capacitors C_gs and C_gd in a JFET are due to reverse biased gate.

i. The capacitance between the source and the gate C_gs is similar to the collector base capacitance C_b'e and the capacitance between drain and gate is similar to emitter base capacitance C_b'e

This equivalent circuit can be modified using Miller's Theorem as shown in Fig.2.32.

The input/output feedback capacitance C_gd is replaced by input/output shunting capacitance similar to BJT analysis.

The Thevenin's equivalnt circuit for the above circuit is obtained as shown in Fig.2.33.

Applying voltage division rule,

The output voltage,

Applying voltage division rule to the input circuit

Where ω₂₁ = upper 3 dB cutoff frequency due to input circuit

Overall voltage gain,

The lowest value of ω21, ω22 is selected as upper 3 dB frequency of the amplifier.