# How do we determine alternative model of certain opamp/transistor in a circuit

Discussion in 'General Electronics Chat' started by kevin0228ca, Jun 10, 2015.

1. ### kevin0228ca Thread Starter Member

Jun 5, 2015
30
0
I am beginning to study electric engineering.
I am playing with circuit simulators, sample circuits, and actual circuit I build with parts I have.

My question is that if in a sample circuit use a op amp or transistor model which I dont have.
How do I look if my model is compatible with sample model?
Do I look at data sheet?
for example I am building

but for op amp I am using AD822, and transistor I am using 2SA1015.
But I get Vload = 0.
In simulator with exact model as sample circuit I get correct result.
But simulator does not have AD822 or 2SA1015 so I do not know if my model should work.

Can anyone please provide a concrete example?
Thank you.

Last edited: Jun 10, 2015
2. ### dl324 Distinguished Member

Mar 30, 2015
3,250
626
Have you studied op amp and transistor parameters?

For op amps, things like input impedance, offset voltage, common mode rejection, gain bandwidth, slew rate, output swing (how close it can get to the rails), etc are among the important parameters.

For transistors, beta, breakdown voltages, max collector current, max power dissipation, fT, etc are some of the important parameters.

3. ### dl324 Distinguished Member

Mar 30, 2015
3,250
626
I looked up the specs for your parts and don't see why they wouldn't work. Since you're breadboarding the circuit, have you tried troubleshooting?

What voltage is at the non-inverting pin of the op amp? Is it what it should be? What voltage do you expect on the emitter of the transistor? What would the base voltage be if the transistor was on? Off?

4. ### kevin0228ca Thread Starter Member

Jun 5, 2015
30
0

for R1 = 680, R2 = 10K, R3 = 180

My calculation should give me V+ = V- = 14V and I verified with multimeter.
Therefore V emitter should also be 14V and verified with multimeter.
So current flowing emitter should be (15-14) / 180 = 5.5mA.
and I just fried something while trying to verify 5.5mA with multimeter.

I will test what is fried

5. ### kevin0228ca Thread Starter Member

Jun 5, 2015
30
0
I tested op amp still works.
I verified current flowing emitter to be 5.5mA.

I dont know how to calculate voltage at base.
voltage at base should be 14 - 0.7 = 13.3V?
multimeter gives 3.8V though.

-------------------------------------------------------

I might have my transistor opposite.
I reversed and I think circuit works, at 5.3 mA across varying load.
But why two different datasheet would say opposite pin assignment?

emitter is rightmost if you look at flat side

http://www.unisonic.com.tw/datasheet/2SA1015.pdf
emitter is leftmost if you look at flat side

Last edited: Jun 10, 2015

Aug 1, 2013
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7. ### dl324 Distinguished Member

Mar 30, 2015
3,250
626
The pinouts look the same to me. The first datasheet is showing the bottom view. This sometimes confuses me too and I wish they'd note that it's bottom view. If the creator new anything about mechanical drawings, they'd dash the "hidden" portions. The second datasheet has no ambiguity.

8. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
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The whole premise of negative feedback is to make the circuit behavior insensitive to some variation in parts specs. That opamp-current-sink circuit has heavy feedback, so the transistor specifics almost do not matter. The Hfe could be almost anything, etc.
Any opamp that has a large open-loop gain would also likely work in that circuit.