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# minimum current and size for transistor

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#### mona123

##### Member level 5
I want to make an analog control circuit like a DC comparator with BJT or CMOS, using a differential type of structure. How do I know how much current do I need to to use for biasing and what basis should I choose my device size? Can someone help or point me to any book or example how it is designed from scratch. Thanks.

Generally the amount of current required is related to the response time required for the comparator.
What response time do you require?

Generally the amount of current required is related to the response time required for the comparator.
What response time do you require?

It's not given. I need to compare a bias voltage against fixed threshold and then clamp it to the threshold voltage. Let's say 100ps. Is there an example from scratch? Curious what is the calculation to determine the required current from response time and what device size will i choose? Thanks.

100ps is very very fast.
If the response time required is not given why are you selecting such a small value?

100ps is very very fast.
If the response time required is not given why are you selecting such a small value?

I just pulled it from air since you asked, if you can give some example that will be enough for me, i just want to know the procedure of doing it as I asked in the previous message. Thanks.

The speed is determined by the ON resistance of the transistor and the capacitances of the circuit, including the gate capacitances, drain/source capacitances and trace capacitances. So you need to know what those are for the process your are using before you can do a design.
A larger transistor will have a lower ON resistance to charge the capacitances faster, but a larger transistor also has higher capacitance. So it's a trade-off to optimize the capacitance and the transistor size depending upon how many other transistors the output has to drive and how long the traces are.
You need a good simulation tool that has all the process parameters to do that.

Hi,

The procedure is as follows:
Let's say you want to build the circuit like here:

1. You use minimum length of transistors (are the fastest and takes smallest amount of place).
2. Use any width of transistors as you like. Start for example with 10 um.
3. Take any current you want. For example, 50 uA for current source of differential pair.
4. Simulate the circuit and write down its parameters (power, bandwidth etc.).
5. Compare the parameters with the required spec and adjust your circuit by changing currents values, widths, lengths etc.

Generally, you like to start with small current values, shortest lengths and small widths. Why use a lot of current and take up a lot of space (large L and W)? And then you start tuning if you do not meet specification.

Hope it's helps.

BTW. I think every designer has his own routines, so treat the above just as an example.

Thanks. So what would be the smallest current to start. pA, nA or fA, where is the lowest floor and why is what I want to understand mainly. Thanks.

dozens of uA. 10, 20, 50, 100 so on. I think you need specific amount of current to charge or discharge gate of a transistor in a needed time. If your current is 1 fA and the gate capacitance is 1 fF you need 1 s to change gate voltage by 1 V (1V/1s). And it is not want we want in IC where we operate in us or ns.

mona123

### mona123

Points: 2
Thanks that helps but would it matter if it's BJT versus FET in that case there is no charging of gate capacitance is required, so we can go infinitesimally lower in current?

There are always parasitic capacitances. Additionally, you need a specific amount of current for the base of BJT transistor to have a desired output current. Moreover, speed of bjt depends on their currents. I saw a graph for it in the past, but I canot find it. If I remember correctly, beta also depends on currents.

So long story short, you cannot use 1e-21 current for bjts. They are not ideal.

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