Current Mirror circuits

[EngrPeterO]

Hello. Just recently found this forum and it certainly looks like you guys have a lot to offer. Hopefully, I can do the same moving forward.

In the meantime, I am looking for some additional information on current mirror circuits. I have spent some time reviewing google links for 'current mirrors' and believe I have a pretty good understanding of simple designs. However, I have a circuit 'feeding' two current mirror circuits that I believe to also be a current mirror that is composed of a PNP transistor and an NPN transistor with their bases connected (and grounded). I haven't been able to find much info in the current mirror links I searched through for this configuration. Anyone know of some good links (that don't show up on the fist several pages of a google search!) that might further describe this transistor connection?

Sincerely,
Peter

 

[MikeHalloran]

Look for National's data sheets and app notes for the LM3900.



Mike Halloran
Pembroke Pines, FL, USA

 

[Comcokid]

It's a little hard to get an idea of what the circuit looks like. Could it be a current mirror connected to a current sink?

Another IC with a uncommitted current mirror is the REF200 from TI (a former Burr Brown part). The data sheet has several examples of the connection of the current mirror with current sources/sinks.

 

[EngrPeterO]

Hi Mike. Thanks for the suggestion. I went to National Semiconductor LM3900 and looked over the applications notes, specifically the Application Note AN-72 - which has 46 pages. I scanned the topics and pages for circuits having configurations like the one I am trying to learn something about without luck. Were you thinking of a section of this application note in particular?

 

[EngrPeterO]

Comcokid, my apologies for a poor description of the configuration I don't understand. Hopefully, this description will be better...

Take a pnp transistor symbol - draw it with the base facing right and the emitter facing up. Place an npn transistor symbol to the right of the pnp transistor with the npn transistor's base facing left and emitter facing up. Connect the two bases together (and connect them to ground).

The two emitters are connected by identical resistors to the same point - an output of an op-amp. Each transistors collector is connected to a separate current mirror circuit (those I recognize) by way of a resistor.

Anyway, it is the operation of the back-to-back npn and pnp transistors that I am having difficulty figuring out.

 

[IRstuff]

The base connection to ground makes no sense. Where's the signal? And with base grounded, the NPN is forever off.

Is this from a datasheet that you can point to?

What you seem to be describing is a push-pull output stage, rather than a current mirror.

Having current sources in the output stage also sounds wrong, since current mirrors are usually cannot drive high currents, which an output stage usually does. The collector resistors in output stages are usually intended to provide damping for inductive loads.

TTFN

 

[jimkirk]

Simple description...(assuming the op amp has a positive and negative power supply)

The op amp is just turning one or the other transistor on, depending on the outupt polarity. There will be a bit of a dead band where both transistors are off, when the op amp output is near ground.

What happens on the collector side of the transistors depends on the circuit. When the op amp output is negative, the NPN will try to sink current approximately equal to (Vopamp-0.7)/R. When the opamp output is positive, the PNP will try to source the current. NPN collector compliance is roughly negative to ground. PNP collector compliance is roughly positive to ground.

 

[jimkirk]

Oops, mind the polarity of the Vbe drop in above post. In each case, it subtracts from the magnitude of the opamp output voltage. So the NPN wants to sink (Vopamp + 0.7)/R, where Vopamp is negative.

 

[EngrPeterO]

Ahhhh, that's it, jimkirk. I go so focused on the current mirrors that are after this pnp-npn arrangement, that I incorrectly figured they were doing something similar. The opamp can output bipolar signals (and is intended to output a 'drive' signal to turn one or the other of the post circuit current mirrors on. Thanks for the comments. With your comments and a relook at the circuit, I now see what is going on.

Thank you all for your comments.

Sincerely,
Peter