# Amplifier circuit - VGS value help please

• Engineering

## Homework Statement:

FIGURE 4 shows an amplifier circuit. Write a short report on the
operation and performance of the circuit. In completing the report you
should:
• Explain the operation of the circuit and in particular the role of
resistors R1 and R2.
• Build the circuit in PSpice and use it to determine:
(i) the quiescent value of Vout.
(ii) the voltage gain for a 100 mV, 1 kHz , input signal.
• Sketch the small-signal equivalent circuit of the amplifier and use it
to estimate the voltage gain. Compare your answer with that of (ii)
above.
• Attempt to calculate the quiescent value of Vout. Compare your
answer with that given by the PSpice model. Try to explain any
discrepancies.

## Relevant Equations:

VGS = -IDS RS
I have calculated my own solution but I am struggling to work out how to use Pspice to compare my values. I am using simetrix for this - could anyone help with this part?

My solution is:

VGS = -IDS RS
Av = Vo/Vi
Vo= -gm VGS RL
Vi = VGS (1+gmRS)
gm= IDS/VGS so gm= IDS/-IDS RS = 1/RD = -1/100 = 0.01

so

Av = -gmRL/1+gmRS = -0.01 x 1500/1 + 0.01 x 100 = -7.5
Vo = Av x Vi = -7.5 x 0.1 = -750mV

VDS = VDD - IDS (Rd+Rs)
VD-VS = VDD - IDS Rd - IDS Rs
VD = VDD - IDS Rd
IDS = (VDD - VD)/RD = (12- (-0.75))/1500 = 8.5mA
gm = IDS/VGS, so VGS =IDS/gm = 0.0085/0.010 = 0.85V

I think that is correct, i am just not sure how to use simetrix to check it, any help would be appreciated. Thank you

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berkeman
Mentor
Welcome to the PF.

I think that is correct, i am just not sure how to use simetrix to check it
So if SIMetrix is like most SPICE simulators, you place the parts into the schematic and make the connections with "wires". You add the source (be careful about DC biasing, since it looks like this input is not AC coupled), and then with the source off, you find the DC bias points in the circuit. Then you can do a transient or AC analysis to see what the output/input gain ratio is.

http://bwrcs.eecs.berkeley.edu/Classes/IcBook/SPICE/UserGuide/analyses_fr.html

http://www.csun.edu/~jaf35230/pspice_tutorial_transient.pdf

Can you show us the SPICE schematic you drew and list the DC bias points that were found?

Hi, thanks for the welcome and for taking the time to reply :)

I have built the circuit - I will attempt to attach it. it was simply the analysis part i was unsure about, so thank you for your help there.

Could you explain a bit more about the DC bias points if you have the time? This is my first time using it and some of the terminology is totally alien to me.

I am also not even sure how to turn off the source. I realise this is real beginner stuff but the course notes only showed how to build the most basic of circuits and do a simple voltage probe

I've attached the graph it gave me when i added some probes in.

But I am still not quite sure how to analyze the JFET itself? I don't seem to be able to select it for analysis?

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