# bjt amplifier in pspice

#### Hooman95

Joined Mar 4, 2014
3
Hello. I'm going to design a simple bjt amplifier with a CC stage and a CE stage in Pspice.
But, How can psice calculate the gain, I mean in a way exept of looking at the out put graph and guess the voltage gain.
How could I do AC sweep?
Can anyone give me a spice toturial for amplifiers? because most of pspice toturials I've found, generally discuss about RC circuits, make a new project,..

thanks

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#### crutschow

Joined Mar 14, 2008
25,991
You can calculate the gain by dividing the output AC voltage by the input source AC voltage. No guessing involved. (I assume there are cursors available for the plots that give you the exact voltages). If the plots allow waveform arithmetic then you can plot V(out)/ V(in) where out and in are the output and input nodes respectively.

If you want the gain versus frequency then do an AC analysis over the frequency range of interest.

#### shteii01

Joined Feb 19, 2010
4,644
A more basic ilustration:
Output=Gain*Input
You know the Input, because you set it up.
You know the Output, because you measured it.
Solve for Gain. What could be simpler?

#### mvaseem

Joined Jan 31, 2014
48
If you want to use AC analysis for gain measurement, do following -

- Replace V3 source in your schematic with AC source (VAC from source.olb)
- Do AC analysis in the frequency range of interest. Go to following link to see how to do AC analysis (page 322)- http://www.ing.unitn.it/~fontana/spiceman/AC_Analysis.pdf

- Plot the output node. Since AC input amplitude is 1 by default, the output (at low freq) would give you gain. You can also figure out the bandwidth of amplifier by that method.

If you want to use transient analysis for gain measurement, do following -

- Replave V3 by VSIN from source.olb. Provide small signal amplitude (like 1mV), freq of interest, Offset (typically 0).

- Do transient simulation for time to have couple of cycles. Following link - http://www.csun.edu/~apr69082/PSpice/bjttransient.html
- Plot output node.
- Divide output amplitude with input amplitude to get gain.

#### Hooman95

Joined Mar 4, 2014
3
Hi mvaseem. your guides was helpful. the pages you attached seems to be chosen from a good book. woud you send me a link to download the book?
thanks

#### Hooman95

Joined Mar 4, 2014
3
I mean without using corsurs and deviding output amplitude over the input, how pspice itself computes the voltage gain?

#### LvW

Joined Jun 13, 2013
1,160
PSpice is a circuit simulation program - and as such it calculates voltages and currents.
However, the display unit (PROBE) is able to show the ratio of two such quantities.
If you know how the term "gain" is defined, it should be possible to display the wanted ratio.

#### mvaseem

Joined Jan 31, 2014
48
Hi Hooman,

I just did the google search for the links. One of them is basically a pspice user guide. If you have pspice installation, it would be located at - <install>/doc/pspug/pspug.pdf.

For AC analysis, since input AC manitude is 1, pspice would give you gain by directly plotting Vout.
There are predefined templates also available in pspice. The template - "Bode Plot dB" would give you gain and phase for Vout.

You can also create measurements and macros where you can save all the calculated (divide traces, multiply etc) expressions and just reload them after simulation.

-mvaseem

#### crutschow

Joined Mar 14, 2008
25,991
As mvaseem noted, if you use a value of 1 for the source in an AC analysis then the output directly equals the gain in dB since the output is plotted with a reference value of 1 for the dB calculation (or dBV).