# Gain of a transistor

1. May 23, 2010

### Danish_Khatri

I am studying the design of an amplifier in common emitter configuration. I am studying from microelectronic circuits by sedra and smith. the author has described two terms for gain of a amplifier. one is the represented by Av and is called the voltage gain of that amplifier and the other is Gv called the overall gain of amplifier. I m not able to understand the difference between the two. can anybody help me out with this confusion?

2. May 23, 2010

### sophiecentaur

Voltage gain just says what it says. It is the ratio of the volts out over the volts in.
'Overall gain' will be referring to the gain in actual Power, that is achieved. After all, a transformer can give you a voltage gain of 100, but the power transferred can't be greater than unity.
It's a matter of input impedance and output impedance. For instance, if you want to feed 100V into 50Ω you need 200W. To feed 100V into 1kΩ you only need 10W.

3. May 23, 2010

### Danish_Khatri

Dear I was not able to grasp on your answer. would you please explain it in terms of transistor itself

4. May 23, 2010

### waht

Depending on the edition that you have, in the fifth edition Sedra/Smith on page 463 the quantities G_v and A_v are clearly defined.

G_v is the ratio of v_out to the open voltage of the source generator (it's not the input voltage of the amplifier).

A_v is the ratio of v_out to the input voltage of the amplifier.

Last edited: May 23, 2010
5. May 23, 2010

### sophiecentaur

A fair enough question but you need to realise that a transistor never operates in isolation. It is part of an amplifying circuit and it has no inherent Voltage Gain without being in a circuit - you actually state your question in the context of a common emitter circuit so you appreciate what I have just said.

6. May 25, 2010

### Danish_Khatri

Thanks for your help dear. I understood this concept when I repeated the paragraph.