How Can I Use Simetrix to Verify My Amplifier Circuit Calculations?

In summary: The schematic I drew showed the DC bias points and the analysis results. I don't think you can select the JFET for analysis, but you can select other components in the circuit for analysis.In summary, the author has calculated their own solution to a problem, but is struggling to understand how to use Pspice to compare their values. They are using Simetrix to do the analysis, but are not sure how to turn off the source or select the JFET for analysis.
  • #1
peasngravy
72
6
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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  • #2
Welcome to the PF. :smile:

peasngravy said:
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?
 
  • #3
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 realize 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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Last edited:

1. What is the VGS value in an amplifier circuit?

The VGS value in an amplifier circuit refers to the voltage between the gate and source terminals of a MOSFET (metal-oxide-semiconductor field-effect transistor). It is an important parameter that determines the amount of current flowing through the MOSFET and ultimately affects the amplification of the circuit.

2. How is the VGS value calculated?

The VGS value can be calculated using Ohm's law, by dividing the voltage across the gate and source terminals by the resistance between them. It can also be determined using the MOSFET's datasheet, which provides the VGS value for various operating conditions.

3. What is the ideal VGS value for an amplifier circuit?

The ideal VGS value for an amplifier circuit depends on the specific MOSFET being used and the desired amplification. It is important to ensure that the VGS value falls within the MOSFET's specified range to avoid damaging the component or affecting the performance of the circuit.

4. How does the VGS value affect the amplification of an amplifier circuit?

The VGS value affects the amplification of an amplifier circuit by controlling the amount of current flowing through the MOSFET. A higher VGS value results in a larger current and therefore a higher amplification, while a lower VGS value leads to a smaller current and less amplification.

5. What should I do if the VGS value is too high or too low in an amplifier circuit?

If the VGS value is too high or too low in an amplifier circuit, it can be adjusted by changing the input voltage or using a different MOSFET with a different VGS value. It is important to ensure that the VGS value falls within the specified range to avoid damaging the component or affecting the performance of the circuit.

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