Electronics > Beginners
Common Collector Question
eev_carl:
Hi,
I was trying to breadboard the attached DC common collector circuit, but haven't been able to get the same results as I do with LT Spice. I expected to get the 2.5 - 0.7 = 1.8 but I get values closer to 2.1. I've tried a few breadboards and PN2222As so I think the components are ok. Also, when I apply AC (with a bias network and filter cap), the emitter follower looks fine on the scope.
Is this within the error range with my noisy breadboard setup?
Thanks,
Carl
xavier60:
You have possibly made an oscillator. I was able to make a BC548 oscillate at 100Mhz by putting a bypass capacitor from Base to ground and 20cm of unbypassed wire in series with its Collector.
eev_carl:
I think you're right. It seems like my multimeter is affecting the circuit. When I scoped the emitter pin, I got a flat line of 1.8V. However, when I added the multimeter (with the scope), it read 2.1V and the signal was distorted on the scope.
Is it wrong to use a DC multimeter to read this voltage?
xavier60:
--- Quote from: eev_carl on September 28, 2019, 12:14:17 pm ---I think you're right. It seems like my multimeter is affecting the circuit. When I scoped the emitter pin, I got a flat line of 1.8V. However, when I added the multimeter (with the scope), it read 2.1V and the signal was distorted on the scope.
Is it wrong to use a DC multimeter to read this voltage?
--- End quote ---
Not really wrong to use a multimeter, just need to be aware that the lead loading can affect some types of circuits like stopping oscillators or causing amplifiers to oscillate. There are likely other contributing reasons caused by circuit layout.
Try taking the voltage reading with a 10K resistor tied in series with the probe. Or any value from 1k to 100K.
TimFox:
Back in the day, the "VTVM" typically used a 1 megohm resisistor in the probe as part of the 11 megohm total input resistance for DC voltage measurements. This isolated the capacitance of the meter and probe cable from the node being measured for exactly this reason. The other common "VOM" passive voltmeter often caused a similar problem to yours when connected to a node with substantial AC capability (as opposed to the power rails or similar).
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