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| BJT current mirror/amplifier |
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| Solstickan:
Hello, I am having some difficulties with an assignment I was given. I'm supposed to determine the values for R1 and R2 such that the currents I1=1 mA and I2=10 mA. The only parameters given is that VCC=+5 V, Vee=-5 V, UBE=0.7 V and the transistors has a high beta value. The voltage drop across R2 can be omitted. I've been working at this all day, but I have not for the life of me figured out how to get a determine the values for R1 and R2 if you don't know the value of R3. The current I2 is going to depend on the value of R3, and for all we've learnt this type of configuration is going to allow the current I2 to be less than or equal to the reference current I1, not higher than the reference. The only progress I've been able to make is through KVL that R1 is approximately 9.3 kOhms to get I1 to be 1 mA. But after that it stopped short, I'm really confused as to how you should find the resistor values without knowing the value of R3. Any hints or tips would be greatly appreciated! Thank you. |
| Marco:
Just say R2 value is large enough to drive T2 into saturation, 1 kOhm would do for instance, R3 being approximately 100 Ohm to generate the 10 mA ... because otherwise you'd have to use a large signal model for the transistor to even begin to calculate this and you haven't been given enough parameters to do that. In theory you could use the small signal model for transconductance together with integration to get a highly simplified large signal model to determine R2, but ehhh. |
| magic:
--- Quote from: Marco on October 07, 2019, 05:46:43 pm ---Just say R2 value is large enough to drive T2 into saturation --- End quote --- :-DD That may actually do ;D But something is telling me that the expected solution was to use the Ebers-Moll equation to calculate how much voltage needs to be dropped on R2 such that the two different VBEs cause the desired currents to be produced. But as Marco says, you would probably need to know some parameters of the transistors. Or maybe they will cancel out once you solve the equations? In such case R2 voltage is very significant, but R3 isn't as long as R3 is less that 100Ω. |
| rfeecs:
Clearly VBE can't be the same for both transistors. The circuit is similar to, but a little different from a Widlar current source: https://en.wikipedia.org/wiki/Widlar_current_source Consider how much VBE needs to increase to give a 10x increase in current. This gives the voltage across R2 so that should allow you to find the solution. |
| Marco:
The formula for that is nonlinear though. If I2 was something like 1.1 ma you could at least use the small signal model for gm ... it's a strange problem, it feels like it should be a simple operating point problem based on extremely simplified assumptions, but isn't. |
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