Seeking Help with Gain Calculation

  • Thread starter Thread starter raju20
  • Start date Start date
  • Tags Tags
    Calculation Gain
Click For Summary

Discussion Overview

The discussion revolves around the gain calculation of a specific amplifier circuit, focusing on a JFET differential amplifier configuration. Participants explore the circuit's design, components, and stability, while addressing the challenges faced by a student preparing for exams.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Raju seeks assistance in calculating the gain of an amplifier circuit and expresses difficulty in understanding it.
  • One participant questions the stability of the amplifier circuit and asks if it has been built or analyzed.
  • Raju identifies the circuit as a Differential FET cascode configuration but lacks knowledge about FET cascode operation.
  • Another participant suggests that the 100G resistor in the schematic is likely for simulation purposes and questions the use of an under-biased LED for biasing.
  • A later reply proposes that the LED might be used for temperature compensation, referencing its application in transistor current source designs.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the circuit's components and their purposes, with no consensus reached on the stability or design choices of the amplifier circuit.

Contextual Notes

Participants note uncertainties regarding the circuit's stability, the appropriateness of the components used, and the implications of the LED biasing method. There are also unresolved questions about the application of gain calculations specific to JFETs compared to BJTs.

raju20
Messages
2
Reaction score
0
Hi,

My name is raju and I am from India. I am in my first year of Degree doing Electronics and Electrical Engineering. I am having a bit of problem with this circuit I am attaching the circuit diagram.

Can anyone please tell me what is the gain of the amplifier and how can I found it. Please i would really appreciate if anyone can help me out. I am about to have my exams and i am preparing but i just can't figure out this circuit please help me..
 

Attachments

  • high impedance amplifier.jpg
    high impedance amplifier.jpg
    33.9 KB · Views: 515
Physics news on Phys.org
Welcome to the PF, raju. I've moved your post from the EE forum to here in the Homework Help forums.

How would you go about analyzing this circuit? What can you tell us about the various parts of the circuit? Why in the world is an LED being used (at very low current BTW) for the input stage current bias setting?
 
And the overall amplifier circuit does not appear to be very stable -- has this circuit been built or analyzed?
 
Hi,
No I have not build this circuit. I was searching on the internet finding some examples regarding JFET amplifiers when i came across this. I think that it is a Differential Fet cascode configuration in which one of the transistor is working as a voltage buffer and the other as a current buffer. I know how to work out the differential amplifier gain for BJT and I think same can be aplied for JFETs but i dnt have that much idea about the FET cascode. I am sorry i dnt know why he used an 2N4401 and specially the 100G resistor I have never seen someone using a 100G resistor.
 
raju20 said:
Hi,
No I have not build this circuit. I was searching on the internet finding some examples regarding JFET amplifiers when i came across this. I think that it is a Differential Fet cascode configuration in which one of the transistor is working as a voltage buffer and the other as a current buffer. I know how to work out the differential amplifier gain for BJT and I think same can be aplied for JFETs but i dnt have that much idea about the FET cascode. I am sorry i dnt know why he used an 2N4401 and specially the 100G resistor I have never seen someone using a 100G resistor.

The 100G resistor is just in the schematic for simulation purposes, most likely. It certainly is not a physical component. I also don't like the under-biased LED being used in the BJT bias circuit -- that's really goofy, unless there is some magic reason for using it.

Do you have access to a SPICE program? You could try simulating the circuit, although like I said, it sure doen't look like it has a very stable feedback arrangement. I doubt that the output opamp will appreciate all the phase shift and delay from having the diffamp in its feedback path...
 
I know this is an old post but I think the LED as the bias is for temperature compensation. I have seen this in transistor current source applications, never used it myself but the led junction has the opposite TC as the Vbe junction. Well almost.

re: From WIKI

http://upload.wikimedia.org/wikipedia/en/e/e8/Const_cur_src_113.gif
 
Last edited by a moderator:

Similar threads

Engineering Diff gain of a push pull degenerated differential pair
  • · Replies 6 ·
Replies
6
Views
3K
How to Calculate the Complex Op Amp in Circuit Diagram
  • · Replies 8 ·
Replies
8
Views
3K
Engineering Designing an Integrator Circuit with Given Absolute Gain
  • · Replies 5 ·
Replies
5
Views
2K
Engineering How can I find the open loop voltage gain of this opamp?
  • · Replies 9 ·
Replies
9
Views
3K
Calculating Required Gain for Fifth Intermediate Amplifier
  • · Replies 3 ·
Replies
3
Views
2K
LTspice: Implementing a Single Balanced BJT Mixer
  • · Replies 17 ·
Replies
17
Views
3K
Engineering Link Budget Antenna: use of effective aperture and the gain in a problem
  • · Replies 3 ·
Replies
3
Views
2K
Engineering Use voltage nodal analysis to solve for the current flowing in this AC circuit
  • · Replies 2 ·
Replies
2
Views
2K
Engineering Analyzing this operational amplifier circuit with DC
  • · Replies 8 ·
Replies
8
Views
2K
PLL - How to find all the gains of a PI corrector and fix Ki ? MATLAB
  • · Replies 2 ·
Replies
2
Views
2K