BJT Amplifier Configurations

Question 1

What are the 3 types of amplifiers?

Answer 1

• Common-Emitter (CE)
• Common-Collector (CC)
• Common-Base (CB)

Question 2

Describe the configuration of a CE amplifier.

Answer 2

The collector is connected to V_CC via a resistance R_C.

The emitter is connected to ground via a resistance R_E.

The base is connected to vs (which is grounded) via a resistance R_s.

Question 3

Which small signal model do we use for a CE amplifier?

Answer 3

Hybrid-π model

Question 4

Describe the hybrid-pi model for a CE amplifier.

Answer 4

V_c connected to Base through resistor R_s.

B connected to resistor r_π.

r_π is connected to the emitter which is also connected to a voltage controlled current source.

ro is connected in parallel to to the voltage-controlled current source.

Next, is the collector which is connected to the resistor R_C

Question 5

What does the small signal model allow us to assume for the CE amplifier?

Answer 5

There is an associated capacitance in parallel with the emitter resistance.

The impedance of the capacitor is 1/jωC, where ω = 2πf.

In the small model approx, frequency f >> 0 so therefore the impedance of the capacitor will be negligible. Therefore, we can negate the resistance RE when considering the small signal ac model as it follows the path of least impedance.

Question 6

What is the input resistance for a CE amplifier using the small signal model?

Answer 6

Using hybrid-pi input resistance R_i = r_π

Question 7

What is the ratio of v_π/v_s for CE amplifier (small signal model)?

Answer 7

Vs and v-pi form a voltage divider which obeys the ratio:

v_π/v_s = r_π/(r_π + r_s)

Question 8

What is the output voltage for a CE amplifier (small signal model)?

Answer 8

vo = - (g_mv_π)(R_c || r_o)

Question 9

Describe the configuration circuit for a CC amplifier.

Answer 9

1. The collector is connected to +V_CC
2. The base is connected to vs via a resistor R_S
3. The emitter is connected to a parallel connection of R_E, R_L and C_C.

Question 10

Which small signal model is ideal for the CC amplifier?

Answer 10

Hybrid T Model

Question 11

What assumptions can we make about the CC amplifier when we are considering a small signal?

Answer 11

We can assume that R_E is absorbed into R_L = R_E || R_L

Question 12

Describe the T Model for a CC amplifier.

Answer 12

1. Base is connected to v_s via resistor R_S and there is a voltage vb at the Base node.
2. The collector is connected to ground, and to a current source which is dependent on α and i_e
3. The emitter is connected to a voltage divider which consists of a series combination of resistors r_o and r_e

Question 13

What is the input resistance for a CC amplifier (small signal model)?

Answer 13

R_i = v_b/i_b

Question 14

What assumptions can we make about the resistances for a CC amplifier?

Answer 14

r_e << R_L << r_o

Question 15

What is an important feature of the T-model for the CC amplifier configuration?

Answer 15

There is a voltage divider at the input and at the output. Following the ratios:

• v_b/v_s
• v_o/v_b

Question 16

Describe the circuit configuration of a CB amplifier

Answer 16

• The collector is connected to VCC via a resistor RC
• The base is connected to ground
• The emitter is connected to a current source, which is in parallel with a series combination of C_E, R_S and v_S

Question 17

Which small signal model is ideal for a CB amplifier?

Answer 17

Hybrid-T model

Question 18

Describe the hybrid-T model for a CB amplifier

Answer 18

1. The collector is connected at one end to the output, with a with a resistor R_C which is grounded. The other end is connected to a current source which is dependent on αi_e
2. The base is connected to ground.
3. The emitter is connected to the junction of BC via resistor r_e. Also, v_S and R_S are connected to the emitter

Question 19

What is the input resistance of a CB amplifier (small signal model)?

Answer 19

R_i = r_e

Question 20

Compare the input resistance of the three amp types

Answer 20

1. Common Emitter (CE) - Moderate
2. Common Collector (CC) - Low
3. Common Base (CB) - High

Question 21

Compare the input resistance of the 3 amplifier types

Answer 21

1. Common Emitter (CE) - Moderate
2. Common Base (CB) - Low
3. Common Collector (CC) - High

Question 22

Compare the output resistance of the 3 amplifier types.

Answer 22

1. Common Emitter (CE) - High
2. Common Base (CB) - High
3. Common Collector (CC) - Low

Question 23

Compare the voltage gain for the 3 amp types

Answer 23

1. Common Emitter (CE) - High
2. Common Base (CB) - Dependent on R_S
3. Common Collector (CC) - Near Unity

Question 24

Compare the current gain for the 3 amp types.

Answer 24

1. Common Emitter (CE) - High
2. Common Base (CB) - Approx Unity
3. Common Collector (CC) - High