hi

I have attached a 2 stage ce amplifier spice sim. The voltage on the collector of Q2 is 7.3 vdc
when the cursor is on V(n001). However, the .meas in the error log displays a value of 8.0 vdc.
The collector of Q1 shows 7.3 vdc on the schematic and in the error log.

Why would this be?

thx

RS

 

Why would this be?

The .meas MAX is the peak value, not the average (DC) value.

It would be a lot easier to follow the node voltages if you labeled the ones you want to measure (i.e. Q1c), such as you did for Vin.
Also, if you make a circuit change, all the program generated node designations can change.

 

hi RRSS,
You are asking for MAX values in the .meas.
E

 

Hi RRSS,
To get LTS to print the quiescent voltage of a Node,
Place the Cursor on the node.
Right Click
Select 'Place OP data label'

E

 

Hi RRSS,
To get LTS to print the quiescent voltage of a Node,
Place the Cursor on the node.
Right Click
Select 'Place OP data label'

E

thanks guys

 

I also look at the waveforms to see that they are not distorted then I divide the peak voltages by 2.

 

You can also use the .meas AVG function.

 

You can also use the .meas AVG function.

Thanks guys.

I have another question.

Looking at the error log, irc2 = 13.5 mA.

But Vout / Rout = 690 mV / 1KΩ = .690 mA = 690 μA

Shouldn't these agree?

I must be missing something here!

 

hi RRSS,
The irc2 is the Maximum current through Rout, 690uA is the actual signal component current.
E

 

hi RRSS,
The irc2 is the Maximum current through Rout, 690uA is the actual signal component current.
E
View attachment 297797View attachment 297798

Got it. irc2 is the total current. So 13.37 total mA - 0.69 ac mA = 12.68 dc mA

thanks E

RS

 

Got it. irc2 is the total current. So 13.37 total mA - 0.69 ac mA = 12.68 dc mA

thanks E

RS

Again, thanks all for your help!

I have another question:

Stage 1 Load = RC1║Rin(total)2 = 765Ω
Av(Stage 1) = Stage1 Load /(r'e1 + RE11) = 765/(1.95 + 27) = 26.42

Stage 2 Load = RC2 = 1KΩ
Av(Stage 2) = RC2 / (r'e2 + RE21) = 1K/(1.95 + 27) = 34.53

Vs = 1 mV
Vin = Vs(attenuated) = 985μV
Vout = 985μV * 26.42 * 34.53 985μV = 899 mV

The sim results in a Vout = 690mV. Since the stages are identical that makes
sense that 985μV * 26.42 * 26.42 ≈ 690mV . However, the loads for stages 1
and 2 are different and therfore the gains must be different everything else
being the same.

Can someone please clarify this situation?

Thx

 

hi RRSS,
Hope you can follow all this information, ask if a doubt.
For each Stage it shows >>
AC Analysis, Input impedance & Output impedance.

Note: your Sine Source is 1kHz

E

 

hi RRSS,
Hope you can follow all this information, ask if a doubt.
For each Stage it shows >>
AC Analysis, Input impedance & Output impedance.

Note: your Sine Source is 1kHz

E
View attachment 297844View attachment 297845View attachment 297846

Thanks again E.

Sorry to keep bugging you but I have more questions.

1. What is the dashed line and how do I make it disappear?
2. The input impedance of both stages is approx. 3.19KΩ?
3. The output of both stages is approx. 34Ω?
4. The gain of each stage is approx. 33? That would mean 985 μV * 33 * 33 = 1.07 Volts out
but the sim says 690mV.

 

hi RRSS,
The dashed line is the Phase, to delete it, Right Click on Right Degrees Axis, Menu, Dont Plot Phase
E

 

Hi RRSS,
Z Input more clearly shown, note the ac.dec start/stop settings.
E

 

hi RRSS,
Zout shown in more detail, again note ac.dec
E

 

hi RRSS,
Stage gain is shown dB. approx 30dB.
E

Updated:
This shows the Voltage Gain using the same 1mV input signal, note the gain at 1kHz is only 695.

E

 

hi RRSS,
Stage gain is shown dB. approx 30dB.
E

Updated:
This shows the Voltage Gain using the same 1mV input signal, note the gain at 1kHz is only 695.

E
View attachment 297853

Thanks E this is a big help. I am just surprised at the value of the output impedance.

Based on my reading, I thought that the output impedance of stage 1 was equal
to the load on stage 1. Which I calculated to be

Stage 1 Load = RC1║Rin(total2) = 765Ω

and that the output impedance of stage 2 was

the load on stage 2 which I thought was Stage 2 Load = RC2 = 1KΩ.

Perhaps I am confusing output impedance versus load?

RS

 

hi RRSS,
Checkout this video.
E

 

hi RRSS,
The LTSpice method is the Zout of the transistor Collector, if you want the actual circuit output impedance you have to use the standard equations which include the value of the collector resistor.
E