# Calculating node voltages

This question is somewhat modified from the original which was asked by someone else. Note that there is more to this than meets the eye. The OP07 or OP070xxx is a 30+ year old op amp, the transistor is PNP (valid but unusual) and the transistor is driven in a way which must saturate any normal device AND has 10x as much base current as collector current. join the dots and see if the real question can be adduced for a beginner who does not realise the unusualness of what they have got:

I have 5v coming in on two resistor going into a transistor.

Second problem 3 voltages coming into a op amp whats the voltage at the nodes, do I add them? I'm having some problem figuring out these two circuits.

http://www.flickr.com/photos/79124850@N08/6931030190/in/photostream http://www.flickr.com/photos/79124850@N08/6931030192/in/photostream

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@llc I have reasked your question here in a slightly different form. Anything that you can tell us about the background to this question will help. Where do the orignal diagrams come from? Is this homework? Why an OP070 opamp ? – Russell McMahon Apr 15 '12 at 3:08
I don't think he will be notified since he didn't participate to the post – clabacchio Apr 15 '12 at 8:07

These are both rather unusual circuits.
They appear to come from a very old source.
The diagrams give the impression of possibly being from an analog computer description.

Is this homework or an assignment?
I may have though so, but the transistor circuit is probably not sensible in a normal context - it acts as a hard on off switch and the base drive is vastly higher than ususual.

An OP070 MAY be a variant of an OP07 (eg an OP070000 exists) which is a very very old design (30+ years) op amp that would not usually be used nowadays - so these may be out of an old book.
Datasheet for OP07. Original was 1983 dated

The 3 diodes - Vout will be the highest of the 3 input voltages less a nonconducting diode drop. Here the +15V is highest so V at pin 3 is 15V - Vds or about 15-0.2 = 14.8V. Depends on diode type.

With the transistor circuit the transistor is PNP (suggesting a very old diagram source) and Vin and Vcc is negative - which is correct for PNP normal operation. R5 will pass about 4.5 mA so the transistor will be turned hard on.
Q1 collector = output node will be almost at ground. The transistor will have some "saturation voltage" but it will be very low in most cases. It is VERY unusual to drive a transistor via a base resistor that is 10x smaller than the collector resistor. This makes the transistor turn on very hard indeed and Vsat will be very low.

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