Cascode Amplifier or CE-CB Configuration:

The CE-CB configuration (usually referred to as cascode amplifier) is shown in Fig. 16.44. Figure 16.44 shows a cascode configuration with a common-emitter (CE) stage feeding a common-base (CB) stage. This configuration of course, has basically the input characteristics similar to those of CE amplifier and output characteristics similar to those of CB amplifier. More specifically, it has high output resistance and is inherently more stable. The high output resistance is useful in achieving large voltage gain. This arrangement is designed to provide a high input impedance with low voltage gain to ensure that the input Miller capacitance is at a minimum with the CB stage providing good high-frequency operation.

A practical version of a cascode amplifier is given in Fig. 16.45.

DC Analysis:

In a cascode amplifier depicted in Fig. 16.45 R₁, R₂ and R₃ form a bias network for transistors Q_{1 and} Q₂; C_S1 and C_S2 provide ac signal ground paths from the Q₁ emitter and Q₂ base, respectively; C_in is the coupling capacitor. For the proper operation of the circuit it is necessary that transistors Q₁ and Q₂ must be identical.

For dc conditions all capacitors are assumed to be open circuited.

Applying voltage divider rule, we have

Emitter current of transistor Q₁,

Once the emitter current of transistor Q₁ is known, all other currents in and voltages at the three terminals for Q₁ and Q₂ can be readily determined if required.

AC Analysis:

Figure 16.46 depicts the small-signal T-equivalent circuit for the cascode amplifier given in Fig. 16.45. Here we will determine low-frequency, small-signal properties (like voltage gain, current gain and input resistance) of the cascode amplifier circuit. In low-frequency analysis, all capacitors are considered short elements.

In Fig. 16.46, R_B = R₁ || R₂; r′_e1 ≡ r′_e2 because I_E1 ≡ I_E2; and V_out1 is the output voltage of the CE stage. Since Q₁ and Q₂ are identical transistors, β_ac1 = β_ac2 and β_dc1 = β_dc2 and subscripts will be omitted in the calculations that follow.

1. Voltage Gain: For the input circuit given in Fig. 16.46, since R_B ≫ r′_e1, v_in1 = r′_e1i_e1 and v_out1 = – r′_e2i_e2

Since, i_e2 = i_c1 and i_c1 ≡ i_e1, it means that i_e1 ≡ i_e2.

Voltage gain of CE stage is

The output voltage,

Substituting the value of i_e2 from above equation in Eq. (16.92), we have

Then the voltage gain of CB stage is

Thus overall voltage gain of the cascode (CE-CB) configuration

2. Current Gain: The current gain A_i of the cascode (CE-CB) configuration may be expressed as

Applying the current divider rule to the input circuit, we have

Thus, the current gain Eq. (16.95) becomes

3. Input Resistance: For input circuit,

Substituting the value of i_b1 from Eq. (16.96) in above equation, we have

and Input resistance,

The cascode amplifier shown in Fig. 16.44 is not encountered very often in discrete circuitry because (i) it needs a large number of resistors and capacitors for providing bias and preventing unwanted feedback and (ii) it does not offer much flexibility as the IC cascode amplifier does.