BJT Common Collector Amplifier: Voltage & Current Gain Calculation

  • Thread starter Thread starter gfd43tg
  • Start date Start date
  • Tags Tags
    Bjt Gain
Click For Summary

Discussion Overview

The discussion revolves around the calculation of voltage and current gain in a BJT common collector amplifier circuit. Participants are attempting to analyze the circuit, clarify their understanding of relevant laws, and solve for the gains based on the provided circuit parameters.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about their connections in the circuit and questions the concept of gain without losses, suggesting a need for clarification on the principles involved.
  • Another participant points out the application of Kirchhoff's laws, specifically mentioning the relationship between emitter current, load current, and base current.
  • A participant seeks clarification on what Kirchhoff's laws are, indicating a gap in understanding among some participants.
  • One suggestion involves working backwards from the output current and voltage relationships, assuming Vbe = 0, and emphasizes the importance of certain assumptions in the calculations.
  • Another participant provides an equation for Vout in terms of Iin and suggests a relationship for Vin, but does not provide a complete solution for the gain.
  • Several participants repeatedly question why Vin is not simply calculated as Iin multiplied by Rin, indicating confusion about the circuit's behavior and the application of Kirchhoff's voltage law.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the correct approach to calculating the gains, with multiple competing views and methods being proposed. The discussion remains unresolved with ongoing questions and challenges to the presented ideas.

Contextual Notes

There are limitations in the discussion regarding assumptions made about the circuit, such as the treatment of Vbe and the implications of using Kirchhoff's laws. Some participants may be missing foundational knowledge that affects their understanding of the problem.

gfd43tg
Gold Member
Messages
949
Reaction score
48

Homework Statement



The circuit below is a BJT common collector amplifier. Find both the voltage gain (AV = Vout / Vin) and the current gain (AI = Iout/Iin). As with Problem 1, this may seem hard to parse at first, but it is actually pretty straightforward. Carefully replace the BJT symbol with the model for a BJT we used in class, making sure the base (B), emitter (E), and collector (C) terminals are connected properly, then use any method you want to solve for Vout!
For this problem, you can assume Vin ≫VBE.

Homework Equations





The Attempt at a Solution


I'm not sure if my connections are right. Also, with a BJT, if current and voltage are amplified (according to my solution), what is being reduced? I don't think you can get these gains with no losses incurred.
 

Attachments

  • 3.7.png
    3.7.png
    10.6 KB · Views: 597
  • 3.7 attempt 1.pdf
    3.7 attempt 1.pdf
    173.1 KB · Views: 329
Physics news on Phys.org
Your answer is not correct. You simply forget about I Kirchhoff's law and also about II law.
Ie = Iload = Ib + Ic and if we assume Vin>>Vbe then we have
Vin = Ib*Rin + Ie*Rout
 
I'm not quite following. What is I Kirchoff's law and II law?
 
Or yyou can work backwards:
Vout = Iout*Re
Iout = βIb
Ib = ?
finish the set of equations yourself to get Vout. Remember to assume Vbe = 0.
I would also assume βR_L/Rin >> 1 but your prof may or may not agree.
 
Here is my 2nd attempt, how does it look?
 

Attachments

Wrong again

Vout = Iin*(1 + Beta)*Rout
and
Vin = Ib*Rin + Ib (1 + Beta)*Rout
So the gain
Av = Vout/Vin = ??
 
how is it that Vin is not just Iin*Rin?
 
Maylis said:
how is it that Vin is not just Iin*Rin?

V = iR means: the voltage ACROSS the resistor = the current thru the resistor times the resistance of the resistor.

Is Vin the voltage across Rin?
 
Maylis said:
how is it that Vin is not just Iin*Rin?
Try to do KVL for this loop
 

Attachments

  • 3.7.png
    3.7.png
    12.9 KB · Views: 576

Similar threads

BJT output voltage as function of input voltage
  • · Replies 7 ·
Replies
7
Views
3K
Engineering Analyzing the internal circuitry of IC 741 Op-Amp step by step
  • · Replies 4 ·
Replies
4
Views
6K
Engineering How does BJT work in this circuit?
  • · Replies 4 ·
Replies
4
Views
3K
Help Understanding Current Gain Of A BJT NPN Transistor
  • · Replies 14 ·
Replies
14
Views
4K
BJT common collector Colpitts oscillator
  • · Replies 43 ·
2
Replies
43
Views
9K
Engineering Alternative design for a CE-CC cascade amplifier
  • · Replies 1 ·
Replies
1
Views
4K
Investigating the Effects of a Common Emitter Amplifier
  • · Replies 7 ·
Replies
7
Views
2K
Voltage gain of an emitter follower (BJT Common-Collector)
  • · Replies 5 ·
Replies
5
Views
3K
Understanding BJT Common Collector Circuit: Voltage and Saturation Explanation
  • · Replies 10 ·
Replies
10
Views
2K
BJT amplifier - calculate resistances
  • · Replies 16 ·
Replies
16
Views
3K