Current Mirror Question

steveb

Joined Jul 3, 2008
2,436
I checked it out on an O-scope (tektronix) that my uncle lent me and it's a constant 6.00 Volts coming out.
This is not necessarily a good test to determine if lack of filter caps is causing the issue. Often feedback can occur through the power supply due to poor filtering. The voltage may look fine when you test it, and then later there could be an oscillation when you place the current meter to read Ie2. If you have an O-scope, use that to monitor your collector voltages while you are taking your current readings. And use a 10 uF or 100 uF cap on the power supply at the board, plus another 0.1 or 1 uF capacitor for extra caution.

The main measurement issue is the value if Ie2. It's quite possible that when you put a current meter in that location, the circuit oscillates and changes the reading. Any time I've had strange readings like yours, it was the result of doing quick measurements with a multimeter, rather than a scope. A circuit can oscillate just due to having meter probes attached. DC readings will not be correct if the circuit oscillates.

This is my best guess as to what is happening. If it turns out to not be correct, I think I may be stumped.
 

Thread Starter

blah2222

Joined May 3, 2010
582
This is not necessarily a good test to determine if lack of filter caps is causing the issue. Often feedback can occur through the power supply due to poor filtering. The voltage may look fine when you test it, and then later there could be an oscillation when you place the current meter to read Ie2. If you have an O-scope, use that to monitor your collector voltages while you are taking your current readings. And use a 10 uF or 100 uF cap on the power supply at the board, plus another 0.1 or 1 uF capacitor for extra caution.

The main measurement issue is the value if Ie2. It's quite possible that when you put a current meter in that location, the circuit oscillates and changes the reading. Any time I've had strange readings like yours, it was the result of doing quick measurements with a multimeter, rather than a scope. A circuit can oscillate just due to having meter probes attached. DC readings will not be correct if the circuit oscillates.

This is my best guess as to what is happening. If it turns out to not be correct, I think I may be stumped.
I will check this out tomorrow! What a simple circuit yet such a weird issue. I'm hoping that it might just be the oscillation, if there is any.

Thanks again!
 

Potato Pudding

Joined Jun 11, 2010
688
If you don't have LTSpice please download and install it.
It is free - Thank You Linear Technology even if it is a brilliant marketing idea.

This will let you try working on this without component or wiring concerns.
It also allows providing you a sample circuit.

The attached circuit is reasonably simple current mirror configuration.

2 Points I want you to see from this.

The transistor connected Collector to Base is going to show about .6 volts Vce. That is normal.

Be sure that the Rlimit connected to that transistor is LARGER than the resistance on the other side, preferably at least twice as large.
 

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Thread Starter

blah2222

Joined May 3, 2010
582
Thank you for the help, but as I said in a previous message, I can't install any software on my laptop right now as it is running of a USB stick (ubuntu linux) and it wont let me download anything from the internet.
 

Potato Pudding

Joined Jun 11, 2010
688
Sorry that I missed that earlier message.

Still make certain that Rlimit is larger than the resistance on the other side if you want to see the currents match.

 
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