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# KCC's quizzes AQQ235 about a bipolar common emitter amplifier - a kind proposal from our colleague Martin Walker

Apologize for our non-technical audience since this quiz is more for our FAEs...

A kind proposal from our colleague Martin Walker, ADI Product Marketing Engineer, UK:

Most of our electronic engineers have seen this sort of circuit in their first year study time using a bipolar transistor in its 3 famous basic configurations: common emitter, common collector and common base.

Here above is a common emitter configuration.

Conditions:

• Vcc = 6V
• Vout = 3V
• Tc = 25°C

Q1 is a BJT NPN with current gain β of several hundreds.

RB1 and RB2 are large compared to RC and RS

Questions :

1. Is there enough information to work out the gain of this circuit?
2. If so, what is the voltage gain of the circuit?
3. Is it a good amplifier?
4. If not, what would you do to mitigate for its limitations?

Again, many thanks Martin!

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[edited by: emassa at 2:19 PM (GMT -4) on 22 May 2023]
Parents
1. Is there enough information to work out the gain of this circuit?

Yes, there is enough information to work out the gain of this circuit.

2. If so, what is the voltage gain of the circuit?

The gain of  this circuit can be calculated by RC/Re, where Re is the  internal resistance of the transistor at BE junction.

3. Is it a good amplifier?

It can be used as a very small signal high gain amplifier but it is not stable.  No, it is not  good as amplifier.

4.  If not, what would you do to mitigate for its limitations?

This amplifier configuration is highly  sensitive to temperature variations as Re is temperature dependent.

The gain will also be very high due to low Re and hence would be unstable.

To get over these limitations , a resistor is RE is added from emitter to ground of the transistor.

This Resistor will add negative feedback at emitter and stabilise the transistor but  reduce the gain.

The Gain now would be RC / (RE + Re).

To make the gain independent of Re, RE is made bigger than Re to make its effect negligible , hence getting over temperature variations.

If a higher gain is still required , for AC signals, a capacitor CE can be added across RE, this will bring back the gain to RC/Re.

• Thanks Rajesh for your prompt reply! Correct to say temperature variation is the weak point of this circuit, which can still be seen as an amplifier since it is capable to magnify a few tens of mV (at Vs) to several volts at Vout with very standard BJTs such as BC107, BC108 or BC109. We will hear from others to conclude further...