Single stage amplifier with current mirror biasing

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Discussion Overview

The discussion revolves around the analysis of a single stage amplifier using current mirror biasing, focusing on the calculation of drain currents and small signal parameters for MOSFETs in the circuit. Participants explore the relationships between various parameters, including Vgs, ID, and the effects of channel length modulation.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Participants discuss the need to find Vgs to calculate ID for M2, with some suggesting the use of known equations.
  • There is a question regarding the assumption that M1 and M2 have identical characteristics despite differences in Kn values.
  • One participant states that if ID,M1 is 2mA, then ID,M2 should also be 2mA, while another challenges this by referencing the relationship between k values.
  • Another participant proposes that ID,M2 could be calculated as 4mA based on the relationship between k values.
  • Concerns are raised about calculating small signal parameters when channel length modulation is considered to be zero, leading to potential errors in calculations.
  • There is a discussion about the configuration of the amplifier, with participants exploring the definitions of common source and source degeneration amplifiers.
  • Clarification is sought on the common terminal between input and output in the context of M2, with some participants asserting that no common terminal exists.

Areas of Agreement / Disagreement

Participants express differing views on the characteristics of M1 and M2, particularly regarding their drain currents and the implications of channel length modulation. The discussion remains unresolved on several points, including the exact values of ID and the configuration of the amplifier.

Contextual Notes

Participants note that the channel length modulation is considered equal to zero, which complicates the calculation of output resistance. There are also unresolved assumptions regarding the definitions and configurations of the amplifier components.

Who May Find This Useful

This discussion may be useful for students and practitioners interested in MOSFET amplifier design, current mirror biasing techniques, and small signal analysis in electronic circuits.

CKaiL
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Homework Statement


123.jpg


Homework Equations



ID= kn (Vgs - Vth)^2

The Attempt at a Solution



I asked to find the current I D , M2. [/B]

I think in order to solve this question I need to find Vgs but how do I do that?

RC1 = 2k ohm
R = VDD - Vgs / I D ,M2

VDD = 5V , and R = 2k ohm and I have 2 unknown variable. Vgs and I D ,M2

I think i need to make use of this ID= kn (Vgs - Vth)^2 to solve this question
 
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CKaiL said:

Homework Statement


123.jpg


Homework Equations



ID= kn (Vgs - Vth)^2

The Attempt at a Solution



I asked to find the current I D , M2. [/B]

I think in order to solve this question I need to find Vgs but how do I do that?

RC1 = 2k ohm
R = VDD - Vgs / I D ,M2

VDD = 5V , and R = 2k ohm and I have 2 unknown variable. Vgs and I D ,M2

I think i need to make use of this ID= kn (Vgs - Vth)^2 to solve this question
Good idea! But you must first find ID for M1. Combine your last equation with another expression for ID summing currents at the drain.
Then, you assume M1 and M2 have identical I-V characteristics so knowing IDM1 gives you IDM2, and you're done.
 
Could I have more information why the characteristics are identical for M1 and M2? They only have the same value for Vth while their Kn is different by a factor of 2?

I forgot to mention that I D,M1 is also given as 2mA for this question so I D, M2 is also 2mA?
 
CKaiL said:
Could I have more information why the characteristics are identical for M1 and M2? They only have the same value for Vth while their Kn is different by a factor of 2?
I didn't see that, sorry.
I forgot to mention that I D,M1 is also given as 2mA for this question so I D, M2 is also 2mA?
No. For any MOSFET in the 'saturation' region with λ = 0, which this is since Vgs > Vth & Vds > Vgs - Vth),
I = k(Vgs - Vth)2.
So, since k2 = 2k1, what is I,M2 if I,M1 = 2 mA?
 
Hi

So I find the value for Vgs by subbing in Vth and I D,M1. Once I gotten the value of Vgs , i sub this value into the equation to find i D,M2 with a k2? Am I correct?
 
CKaiL said:
Hi

So I find the value for Vgs by subbing in Vth and I D,M1. Once I gotten the value of Vgs , i sub this value into the equation to find i D,M2 with a k2? Am I correct?
No point in solving for Vgs. From my last post it should be obvious that I_M2 = 2I_M1. Solve for I_M1 as I discussed earlier.
 
Sinc k2 = 2k1 , I2 = 2I1. I2 = 4mA?
 
Last edited:
I have another question for this. I am asked to find the small signal parameter of the 2 MOSFET .
The formula are
Output resistance in the small signal equivalent circuit of a MOSFET operating in the saturation region is given as R0 = 1 /(channel length modulation * Current ID)

I have ID value for both 1D M1 and 1D M2 but in this question , channel length modulation for both are equal to 0
when you sub in these value , you will get an error.
 
CKaiL said:
Sinc k2 = 2k1 , I2 = 2I1. I2 = 4mA?
Good guess!
 
  • #10
CKaiL said:
I have another question for this. I am asked to find the small signal parameter of the 2 MOSFET .
The formula are
Output resistance in the small signal equivalent circuit of a MOSFET operating in the saturation region is given as R0 = 1 /(channel length modulation * Current ID)

I have ID value for both 1D M1 and 1D M2 but in this question , channel length modulation for both are equal to 0
when you sub in these value , you will get an error.
Suppose the channel modulation was a very small number but not zero - what would be the output resistance then?
 
  • #11
hmm good guess meaning is ID , M2 = 4mA correct or wrong?

For channel length modulation
In the picture it says that the channel length modulation is equal to 0 for channel length modulation M1 and channel length modulation M2.
 
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  • #12
I would also like to check if this is a configuration of a Common source with source degeneration single stage amlifer?
 
  • #13
CKaiL said:
hmm good guess meaning is ID , M2 = 4mA correct or wrong?
Correct.
For channel length modulation
In the picture it says that the channel length modulation is equal to 0 for channel length modulation M1 and channel length modulation M2.
I know. I asked you what kind of number you would get if the channel modulation was a small number but NOT zero.
 
  • #14
CKaiL said:
I would also like to check if this is a configuration of a Common source with source degeneration single stage amlifer?
What terminal is common to the input and output?
What is the definition of "source degeneration"? You want answers on a platter and we're not allowed to give you that - which is to your benefit.
 
  • #15
The source is connected to the gate and Output is connected to the drain
 
  • #16
CKaiL said:
The source is connected to the gate and Output is connected to the drain
 
  • #17
Yes, but what terminal is common to the input and the output? What is the input referred to, and what is the output referred to?
 
  • #18
Input is connected to the gate , Output is connected to the drain. Source is connected to neither input or output
 
  • #19
I am looking at the amplifier at M2
 
  • #20
CKaiL said:
I am looking at the amplifier at M2
Good.
An input is a voltage. A voltage is always between two points.
An output is also a voltage. Same thing.
So, again - what terminal is common to input and output?
 
  • #21
There don't seem to be a common terminal between the input and output if I look at M2 only.The only common terminal I see between the input and output is between M1 and M2 terminal where the body of the M1 (output) is connected to the input of M1
 
  • #22
CKaiL said:
There don't seem to be a common terminal between the input and output if I look at M2 only.The only common terminal I see between the input and output is between M1 and M2 terminal where the body of the M1 (output) is connected to the input of M1
Talking M2 only. Re-read what I said about how voltages always need two points.
The input is between G and S.
The output is between D and S.
See any common terminal yet?
 

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