# Prober probe placement

Discussion in 'Homework Help' started by Fluke289, Apr 15, 2014.

1. ### Fluke289 Thread Starter New Member

Aug 8, 2013
21
3
Im a bit confused on my probe placement to measure this amplifier. I built the circuit on my bench and I want to measure the AC emitter voltage of the second stage of the amplifier, do I have my oscilloscope (XSC1) probes in the correct location? Im not sure if I want channel 1 before or after R11.

For measuring the DC collector voltage for the second stage, do I have my DMM (XMM1) meter probes in the correct location?

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2. ### #12 Expert

Nov 30, 2010
17,840
9,178
Move your scope to the top of R11.
Your DMM is correct.

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3. ### vk6zgo Active Member

Jul 21, 2012
677
85
Let's tell the OP why!

The Reactance of C6 at signal frequencies is so low that C6 in parallel with R12 looks like a short circuit,so there will be no AC voltage across R12.

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4. ### Fluke289 Thread Starter New Member

Aug 8, 2013
21
3
Thanks for your help on the other circuit. I have this circuit here that Im measuring. Im supposed to measure the DC voltage of VCE. When I measure the DC voltage drop of VCE, I assume I turn the function generator off.

When I measured the DC voltage drop of VCE, I got about 1.8V, when I ran a simulation of the circuit, I got a 1.6, and when I calculated it, I got about 1.44V. Is there usually that much variation from calculated and actual measurement of voltage drop? Am I doing something wrong? Am I placing my probes in the wrong spot?

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5. ### #12 Expert

Nov 30, 2010
17,840
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That little discrepancy is about the gain of the transistor. This is not very predictable from one transistor to the next and changes with temperature.

(That means, "yes". There is that much variation.)

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6. ### Fluke289 Thread Starter New Member

Aug 8, 2013
21
3
They taught us about transistor beta, but I didnt realize that it would have that much effect with this particular amplifier design. I was under the impression that this design was to set so that all transistors would work about the same because the beta varies from 100 to 300. I may have misunderstood what they were saying, or it was in reference to another circuit design.

7. ### #12 Expert

Nov 30, 2010
17,840
9,178
Just look at the circuit in a DC frame of mind. What is in there that can change? Not the resistors. The only part that "depends" is the saturation voltage of the transistor and the gain of the transistor. When you try to run a transistor with very little voltage from collector to emitter, the gain gets smaller than advertised. Look at the graph of the transfer curves to see that relationships get distorted at those levels of voltage.

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