Help with BJT constant current source - Page 1
Forum section: electronics / beginners

author: willfly
author_rank: Newbie
timestamp: March 08, 2022, 08:33:01 pm
content: Hello, I tried this simple two transistor constant current circuit, and it oscillates from 20-60Mhz measured across R5 depending on transistor. The circuit is fairly standard and referenced all over the web, but nobody mentions instability. I have tried different transistors - high voltage, RF, PNP, changed breadboards, changed power supply etc but nothing fixed it. Oscillations stop if bypass capacitor is used across emitter resistor R4.I am trying to understand why it is unstable. Do you guys see oscillations in your similar circuit as well?SPICE simulation shows a good phase margin.Thanks for your help.

author: TimFox
author_rank: Super Contributor
timestamp: March 08, 2022, 08:50:14 pm
content: Assuming that your simulation is correct, yet the practical circuit oscillates, I would try adding a small (10 to 100 ohm) resistor in series with the base lead of Q2.Does your SPICE simulation have correct values for device capacitances in the two transistors?

author: Benta
author_rank: Super Contributor
timestamp: March 08, 2022, 08:59:58 pm
content: There's a heck of a lot of gain in that circuit, and I'm not surprised that it oscillates. I've seen that before.What's the purpose of V2 and R5?

author: magic
author_rank: Super Contributor
timestamp: March 08, 2022, 09:07:47 pm
content: V2 looks like an attempt at simulating loop gain, although I would put it in series with Q3 base. I'm not really sure if it is even correct right now.Did you try it on something other than a breadboard? Did you try adding breadboard parasitics to SPICE?I suppose a small capacitor across B-C of Q3 or from its collector to ground may also help, perhaps with less sacrifice of high frequency impedance than bypassing R4.

author: willfly
author_rank: Newbie
timestamp: March 09, 2022, 03:12:01 am
content: Thanks for the replies; here are the observations form your recommendations.Quote from: TimFox on March 08, 2022, 08:50:14 pmAdding base resistor does get rid of the oscillations. The transistor model is what I got from ONSEMI. I also tried with some additional capacitance B-E and B-C. No oscillations in simulations - aka gain margin was poor, but phase margin was 60-240 deg.Quote from: Benta on March 08, 2022, 08:59:58 pmV2 is sweeped to get the AC closed loop response. R5 is a shunt to measure the constant-current. Oscillations happen with or without R5.Quote from: magic on March 08, 2022, 09:07:47 pmI get a similar Bode plot when V2 is in the base of Q3. The phase margin is between 90 - 180 degrees. This should be within the stable closed loop operating region. Adding a small capacitor to B-C of Q3 does get rid of the oscillations as well.I am still unable to understand why this circuit should be anything but stable. It is no different from a common emitter amplifier followed by an emitter follower. Emitter follower is at unity gain, and the Q3 is providing it negative feedback, which should stabilize it completely. My setup is in a noisy environment, so there is a possibility that some RF signal is being picked up from nearby instrument. I will solder the circuit on a veroboard and check again. If possible, can you please try breadboarding it and see if you get similar results.Many thanks.