**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 R_{1}, R_{2 }and R_{3 }form a bias network for transistors Q_{1 and }Q_{2}; C_{S1} and C_{S2} provide ac signal ground paths from the Q_{1} emitter and Q_{2 }base, respectively; C_{in }is the coupling capacitor. For the proper operation of the circuit it is necessary that transistors Q_{1} and Q_{2 }must be identical.

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

Applying voltage divider rule, we have

Emitter current of transistor Q_{1},

Once the emitter current of transistor Q_{1} is known, all other currents in and voltages at the three terminals for Q_{1} and Q_{2 }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} = R_{1} || R_{2}; r′_{e1} ≡ r′_{e2} because I_{E1} ≡ I_{E2}; and V_{out1} is the output voltage of the CE stage. Since Q_{1} and Q_{2 }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′_{e1}i_{e1} and v_{out1} = – r′_{e2}i_{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.