Biasing a MOSFET with constant-current source

In summary, the conversation discusses the issue of biasing a MOSFET with a constant current arrangement. The MOSFET saturation equation is used to determine the required Vgs value for a specified current I. In this arrangement, the gate is grounded and the current source pulls the source voltage negative until Vgs is sufficiently positive for the desired current to flow. If the current source is replaced with a resistor, it would act as a pull-down resistor and the current would need to be calculated. An ideal current source produces the specified current regardless of the voltage across it, so it adjusts Vs to achieve the desired current. The exact value of Vs depends on the set current of the source.
  • #1
CoolDude420
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Homework Statement


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Hi,

I'm just going through amplifier circuits and I'm having some trouble with amplifiers in which the MOSFET is biased by a constant current arrangement. Here's my issue:

In the MOSFET saturation equation, if we set ID, we will be able to obtain Vgs. Which means that if we plug in the value of I into that saturation equation, it will tell us what Vgs value we need to have between the gate and source for us to achieve that specified current I as Vgs determines the resistance of the MOSFET internal channel and hence the current.

Now, how come in this arrangement, there is no voltage at the gate? Wouldn't that mean that no channel has been established and hence current could never flow? Or is there assumed to be a DC bias voltage at the gate? If there is a DC bias voltage assumed at the gate, wouldn't it be considered biased by a gate voltage and not by a current source? Or is it assumed that the current source dictates what Vgs bias voltage is required and that whatever bias voltage that current source requires will always be present for it?

I have all these doubts that I can't find answers to.

Homework Equations

The Attempt at a Solution

 

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  • #2
You say "there is no voltage at the gate". Sure there is. The gate is grounded, so Vg = 0V. The current source will pull Vs negative until Vgs is sufficiently positive so that the current I flows through the transistor.
 
  • #3
phyzguy said:
You say "there is no voltage at the gate". Sure there is. The gate is grounded, so Vg = 0V. The current source will pull Vs negative until Vgs is sufficiently positive so that the current I flows through the transistor.
So the -Vss at the bottom will cause our Vgs = Vg-Vs to become positive just enough to allow our specified I to flow. How is it possible to dictate how much of that -Vss we get? I mean shouldn't we be getting all of that -Vss at the source then our Vgs would be equal to +Vss and then some really high current could flow?

One more thing, if we replaced that current source with a resistor would that be called a pull-down resistor?
 
  • #4
CoolDude420 said:
So the -Vss at the bottom will cause our Vgs = Vg-Vs to become positive just enough to allow our specified I to flow. How is it possible to dictate how much of that -Vss we get? I mean shouldn't we be getting all of that -Vss at the source then our Vgs would be equal to +Vss and then some really high current could flow?

That's what a current source does. It adjusts the voltage until you get the specified current.

One more thing, if we replaced that current source with a resistor would that be called a pull-down resistor?

Yes. In that case, you'd have to calculate how much current would flow.
 
  • #5
phyzguy said:
That's what a current source does. It adjusts the voltage until you get the specified current.
Yes. In that case, you'd have to calculate how much current would flow.

I always thought that an ideal current source produces the specified current no matter what the voltage across it is.
 
  • #6
CoolDude420 said:
I always thought that an ideal current source produces the specified current no matter what the voltage across it is.

It does. Note that we are saying the same thing. In the top part of the circuit (the MOSFET and everything) the current depends on Vs. So the ideal current source adjusts Vs until it gets the current I. This changes the voltage across the ideal current source, which is (Vs-Vss). Ideally, the current I is independent of (Vs-Vss), so it is free to adjust Vs to be whatever is needed.
 
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  • #7
phyzguy said:
It does. Note that we are saying the same thing. In the top part of the circuit (the MOSFET and everything) the current depends on Vs. So the ideal current source adjusts Vs until it gets the current I. This changes the voltage across the ideal current source, which is (Vs-Vss). Ideally, the current I is independent of (Vs-Vss), so it is free to adjust Vs to be whatever is needed.

Ah. I understand now. So in this case, the current source will adjust Vs since it can't change -Vss. And it will adjust Vs to probably some negative value so that Vgs = Vg - Vs = 0 - Vs becomes positive and is greater than the threshold voltage. The exact value of Vs it settles on is dependant on the current I the source is set to.

Would that be correct?
 
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  • #8
CoolDude420 said:
Ah. I understand now. So in this case, the current source will adjust Vs since it can't change -Vss. And it will adjust Vs to probably some negative value so that Vgs = Vg - Vs = 0 - Vs becomes positive and is greater than the threshold voltage. The exact value of Vs it settles on is dependant on the current I the source is set to.

Would that be correct?

Yes, exactly.
 
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  • #9
phyzguy said:
Yes, exactly.
That's perfect. Thank you so much!
 

1. What is a MOSFET and how does it work?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of transistor that is commonly used in electronic circuits. It is a three-terminal device that uses an electric field to control the flow of current between its source and drain terminals. The gate terminal is used to control the electric field, and by extension, the current flow.

2. What is biasing and why is it important for a MOSFET?

Biasing refers to the process of applying a specific voltage or current to a component in order to achieve its desired operating conditions. For a MOSFET, proper biasing is crucial for ensuring that it is operating within its desired range and can amplify an input signal effectively.

3. What is a constant-current source and how is it used to bias a MOSFET?

A constant-current source is a circuit that provides a steady and stable current output, regardless of changes in the load or input voltage. It is commonly used to bias a MOSFET by providing a constant current to the gate terminal, which helps to maintain a stable electric field and ensure proper amplification of the input signal.

4. What are the advantages of using a constant-current source for biasing a MOSFET?

Using a constant-current source for biasing a MOSFET offers several advantages. It helps to minimize variations in the output signal due to changes in temperature or other external factors. It also allows for precise control over the MOSFET's operating conditions, ensuring optimal performance. Additionally, a constant-current source can help to improve the stability and reliability of the overall circuit.

5. Are there any potential drawbacks to using a constant-current source for biasing a MOSFET?

While there are many benefits to using a constant-current source for biasing a MOSFET, there are also some potential drawbacks. One potential issue is that it can be more complex and expensive to implement compared to other biasing methods. Additionally, if the current source fails, it can cause the MOSFET to operate outside of its desired range, potentially damaging the device or affecting the circuit's performance.

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