Download How to perform an AC simulation:

How to perform an AC simulation:
Below is the schematic of an amplifier with the following parameters:
 DC voltage gain =3.5
 AC Voltage gain of roughly -130 (f = 10KHz)
 input Q-point (VINQ ) of 1.2Volts.
You can use either VAC or VSRC as the input voltage source for this type of simulation. (I prefer to use VSRC).
Note the input Q-point, VINQ, is entered as the DC parameter of the Voltage source. The AC parameter is set to 1V;
this is a convenient set up for determining gain parameters. At first glance you might think that this is a problem.
The output voltage at 10KHz (1Vx-130) should surly exceed the power supply. With this type of simulation, Spice
is determining the small signal behavior of the amplifier as a function of frequency. It then takes the computed gain
and multiplies it by 1V (the input AC voltage). For this type of simulation SPICE has no concept of what 130 volts
is. This is NOT true for DC, bias point and transient analysis.
Vcc
Vcc
V2
12Vdc
R4
R1
120k
0
3.5k
Vout
Q1
Vin
Q2N2222
V1
1.7Vdc
1Vac
TRAN =
R3
20k
R2
1k
C1
200u
0
To run the simulation:
1.
2.
Pspice->New Simulation Profile(or edit simulation profile) – enter the name of the simulation if it is new one
You will see the following menu. Choose AC Sweep/Noise as shown. Here SPICE is configured to perform an
AC sweep from 0.1 to 10M Hz. Do NOT make the starting frequency 0. MHz is indicated as 10Meg NOT
10M. The later would be 10mHz (10x10-3Hz).
3.
Hit OK
4.
To run the simulation main menu: Pspice->Run
To Plot the results:
After the simulation is complete you will automatically see a window where you can display the results of your
simulation.
1.
2.
From the menu choose: Trace->Add Trace
You will see the following menu. On the left hand side are all the traces that are possible. On the right had
side are all the functions that you can apply if you wish. Here I will be plotting the output voltage
expressed in dB. Note the output voltage is also the collector voltage of the transistor so V(Vout) abd
VC(Q1), V(Q1:c) are the same thing. You can also plot currents IC(Q1) would be the collector current
IB(Q1) the base current.
To see a plot of the phase use function: P()