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Electronics 2 - Semiconductor Devices > BJT Amplifier Configurations > Flashcards

BJT Amplifier Configurations Flashcards

1
Q

What are the 3 types of amplifiers?

A
  • Common-Emitter (CE)
  • Common-Collector (CC)
  • Common-Base (CB)
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2
Q

Describe the configuration of a CE amplifier.

A

The collector is connected to VCC via a resistance RC.

The emitter is connected to ground via a resistance RE.

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

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3
Q

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

A

Hybrid-π model

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4
Q

Describe the hybrid-pi model for a CE amplifier.

A

Vc connected to Base through resistor Rs.

B connected to resistor rπ.

rπ 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 RC

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5
Q

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

A

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.

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6
Q

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

A

Using hybrid-pi input resistance Ri = rπ

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7
Q

What is the ratio of vπ/vs for CE amplifier (small signal model)?

A

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

vπ/vs = rπ/(rπ + rs)

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8
Q

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

A

vo = - (gmvπ)(Rc || ro)

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9
Q

Describe the configuration circuit for a CC amplifier.

A
  1. The collector is connected to +VCC
  2. The base is connected to vs via a resistor RS
  3. The emitter is connected to a parallel connection of RE, RL and CC.
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10
Q

Which small signal model is ideal for the CC amplifier?

A

Hybrid T Model

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11
Q

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

A

We can assume that RE is absorbed into RL = RE || RL

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12
Q

Describe the T Model for a CC amplifier.

A
  1. Base is connected to vs via resistor RS 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 ie
  3. The emitter is connected to a voltage divider which consists of a series combination of resistors ro and re
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13
Q

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

A

Ri = vb/ib

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14
Q

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

A

re << RL << ro

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15
Q

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

A

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

  • vb/vs
  • vo/vb
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16
Q

Describe the circuit configuration of a CB amplifier

A
  • 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 CE, RS and vS
17
Q

Which small signal model is ideal for a CB amplifier?

A

Hybrid-T model

18
Q

Describe the hybrid-T model for a CB amplifier

A
  1. The collector is connected at one end to the output, with a with a resistor RC which is grounded. The other end is connected to a current source which is dependent on αie
  2. The base is connected to ground.
  3. The emitter is connected to the junction of BC via resistor re. Also, vS and RS are connected to the emitter
19
Q

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

20
Q

Compare the input resistance of the three amp types

A
  1. Common Emitter (CE) - Moderate
  2. Common Collector (CC) - Low
  3. Common Base (CB) - High
21
Q

Compare the input resistance of the 3 amplifier types

A
  1. Common Emitter (CE) - Moderate
  2. Common Base (CB) - Low
  3. Common Collector (CC) - High
22
Q

Compare the output resistance of the 3 amplifier types.

A
  1. Common Emitter (CE) - High
  2. Common Base (CB) - High
  3. Common Collector (CC) - Low
23
Q

Compare the voltage gain for the 3 amp types

A
  1. Common Emitter (CE) - High
  2. Common Base (CB) - Dependent on RS
  3. Common Collector (CC) - Near Unity
24
Q

Compare the current gain for the 3 amp types.

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

Electronics 2 - Semiconductor Devices (7 decks)

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