# small signal model for bjt

#### alimash

Joined Oct 12, 2016
62
dear friends,
i have a general question about bipolar transistor
what is the need of small signal modeling?
thank you

#### MrChips

Joined Oct 2, 2009
22,067
Did you google small signal model?

A circuit designer needs to know two things:

1. What are the DC bias conditions of the circuit,
2. How does the circuit respond to a small AC signal on the input.

Which one do you think is small signal modelling?

• alimash

#### alimash

Joined Oct 12, 2016
62
Did you google small signal model?

A circuit designer needs to know two things:

1. What are the DC bias conditions of the circuit,
2. How does the circuit respond to a small AC signal on the input.

Which one do you think is small signal modelling?
I know it is the second
At class we only use this model to calculate tow things:
-The voltage gain
'the low and high cutoff frequency
My question is:
if the goal was only power amplification;do we need

#### alimash

Joined Oct 12, 2016
62
Did you google small signal model?

A circuit designer needs to know two things:

1. What are the DC bias conditions of the circuit,
2. How does the circuit respond to a small AC signal on the input.

Which one do you think is small signal modelling?
I know it is the second
At class we only use this model to calculate tow things:
-The voltage gain
'the low and high cutoff frequency
My questions are:
-if the goal was only power amplification;do i need to go into the small signal modeling?
-is there any other reason to go into this model?
thank you

#### crutschow

Joined Mar 14, 2008
25,632
-if the goal was only power amplification;do i need to go into the small signal modeling?
-is there any other reason to go into this model?
If you don't use the small-signal model then how would you calculate the power amplifier's gain and frequency response?

#### shteii01

Joined Feb 19, 2010
4,644
When someone says Power Amplification, they mean: amplify voltage and amplify current, at the same time.
What if you want to only amplify voltage?
What if you want to only amplify current?
See? No Power Amplification.
You are going to need a separate model.

• alimash and absf