# bjt amplifier in pspice

Discussion in 'General Electronics Chat' started by Hooman95, Mar 4, 2014.

1. ### Hooman95 Thread Starter New Member

Mar 4, 2014
3
0
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

File size:
41.5 KB
Views:
75
2. ### crutschow Expert

Mar 14, 2008
13,021
3,235
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.

Hooman95 likes this.
3. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,393
497
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?

Hooman95 likes this.
4. ### mvaseem Member

Jan 31, 2014
48
8
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 likes this.
5. ### Hooman95 Thread Starter New Member

Mar 4, 2014
3
0
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

6. ### Hooman95 Thread Starter New Member

Mar 4, 2014
3
0
I mean without using corsurs and deviding output amplitude over the input, how pspice itself computes the voltage gain?

7. ### LvW Active Member

Jun 13, 2013
674
100
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.

8. ### mvaseem Member

Jan 31, 2014
48
8
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

Hooman95 likes this.
9. ### crutschow Expert

Mar 14, 2008
13,021
3,235
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).

Hooman95 likes this.