Common Collector Question - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: September 28, 2019, 11:00:57 am
content: 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

author: xavier60
author_rank: Super Contributor
timestamp: September 28, 2019, 11:54:02 am
content: 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.

author: eev_carl
author_rank: Regular Contributor
timestamp: September 28, 2019, 12:14:17 pm
content: 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?

author: xavier60
author_rank: Super Contributor
timestamp: September 28, 2019, 12:34:25 pm
content: Quote from: eev_carl on September 28, 2019, 12:14:17 pmNot 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.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2019, 01:52:34 pm
content: 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).

author: Audioguru again
author_rank: Regular Contributor
timestamp: September 28, 2019, 03:02:30 pm
content: Of course a Mickey Mouse breadboard causes many circuits to oscillate since it has lots of stray capacitance between its many rows of contacts and its long jumper wires that are all over the place.With very compact wiring (a pcb or compact stripboard layout) and a capacitor to ground at its supply terminal it will work properly.

author: eev_carl
author_rank: Regular Contributor
timestamp: September 28, 2019, 03:54:35 pm
content: Thanks. Adding a 4.k resistor in series with the probe got me the same reading as my ltspice simulation. Is this called DC coupling? Also, does the multimeter have a high impedance that prevents this sort of loading effect?

author: drussell
author_rank: Super Contributor
timestamp: September 28, 2019, 04:13:12 pm
content: The modern multimeter itself has a high input impedance, but that does not negate the effect from the leads themselves causing stray capacitance, etc. when you touch them to your circuit.Adding additional resistance right at the source helps isolate the effects on your circuit from the probe wires themselves while minimally affecting the magnitude of the measurement at the high-z meter itself.As pointed out above, this was precisely the reason that VTVM "probes" had a 1 meg resistor in them. The tube circuits of the day were typically very high impedance compared to what we're accustomed to now (and are often forgotten by today's techs when working on vintage gear) and itoftenmade a huge difference to circuit operation when you tried to "probe" something.

author: xavier60
author_rank: Super Contributor
timestamp: September 28, 2019, 04:13:53 pm
content: The meter itself isn't the problem. The problem is the parasitic reactances of the meter leads. They can appear capacitive at lower frequencies or as an unterminated transmission line and antenna at higher frequencies.The voltage discrepancy is caused by the B-E junction partly rectifying the oscillation signal, called signal bias.Search for Hartley and Colpitts oscillators.

author: eev_carl
author_rank: Regular Contributor
timestamp: September 28, 2019, 11:21:32 pm
content: Note on long leads at end of articlehttps://www.electronics-notes.com/articles/analogue_circuits/transistor/transistor-common-collector-emitter-follower.php

author: GerryR
author_rank: Frequent Contributor
timestamp: September 28, 2019, 11:45:31 pm
content: The circuit you show is a very basic follower circuit. If you bias it using a divider and keep Rb about 1/10 of beta Re (~10k), I don't think you will have an oscillation problem. Just a guess.

author: Jwillis
author_rank: Super Contributor
timestamp: September 29, 2019, 03:27:47 am
content: Did you check that your power supplies are reading accurate . I did the same experiment and got a value 1.94 V at your test point until I checked that the supply .I found out that the readings on the supply are a bit out . I adjusted with an accurate multi meter and got a value of 1.91V at the test point.This gave a a voltage drop of 0.6 V .Rechecked with two other meters and got different test results until I adjusted the supply with each individual meter.With a simple circuit like this I have doubts its an osculation issue but more likely a voltage reading issue.Like most people I can't afford to by the most accurate instruments available and most less expensive ones are always a bit out even the readings on power supplies can be out.Check your supply voltage with the multi meter your using to check your test point. Check your components.Although simulators are good for putting together a circuit they apply data based on whats been input-ed and will almost always give values different from real world applications.Data sheets values are based on an average of several tests but don't reflect the real world values of a particular component.Simulators are based on those data sheets. So you have to allow for a degree of error when you use the components in the real world.Although bread boards do exhibit a parasitic capacitance it tends to be very small. I've tested my boards and found the capacitance to be less than a couple picofarads. This may be a problem for some projects like RF or high frequency experiments but not for simple linear tests like this oneI myself wouldn't worry a lot about small errors to what is expected because this will occur .