PNP cascode current mirror instability oscillations output current issues troubl - Page 1
Forum section: electronics / projects

author: francogomme
author_rank: Newbie
timestamp: March 04, 2025, 03:26:17 pm
content: Hello everyone,I'm working on calibrating a 4-20mA current loop and I am trying to stabilize the output current at 12mA using a cascode PNP current mirror. I’m using BC327-25 PNP transistors (unfortunately, I don’t know the exact manufacturer), and after testing, I found 4 of the transistors have an hFE value around 300.I calculated the reference resistor (Rref) to be approximately 1870 ohms, based on my theoretical design. I assembled the circuit on a breadboard and used a precision ammeter to measure the output current (Iout), as well as the voltages across both Rref and Rload/out.The issue I’m encountering is that the output current fluctuates around ±0.3mA, causing the output voltage to oscillate as well. The reference branch (Rref) is stable with no fluctuations, but the output side shows these oscillations.I’ve double-checked the theoretical values and the design should work, but I’m unable to achieve the stability I need. Could anyone with experience in cascode PNP current mirrors offer insight on what might be causing these oscillations in the output? How can I stabilize the current to exactly 12mA?Any help or suggestions would be greatly appreciated!Thank you!

author: francogomme
author_rank: Newbie
timestamp: March 04, 2025, 03:27:50 pm
content: I wanted to clarify that in my LTspice simulation, I did not include three 47pF capacitors, which are part of the actual physical circuit:One capacitor between the base and emitter of Q1.One capacitor between the collector of Q2 and ground.One capacitor between the collector and base of Q3.I believe the simulation is correct, and it gives a result close to the 12mA output that I am aiming for. However, I would like to emphasize that the issue with the oscillations in the output current arises specifically in the real-world circuit that I built, not in the simulation. The output current in the actual circuit fluctuates by around ±0.3mA, causing the voltage to oscillate, while the reference side is stable.

author: mawyatt
author_rank: Super Contributor
timestamp: March 04, 2025, 03:47:01 pm
content: Q3 as shown has itsEB and C shorted, so the Cap across it has no effect. Sometimes prototypes will oscillate, often due to the added parasitics (capacitance and inductance to a lesser extent), this is especially true with the Prototype Plug-In type boards. Adding a small value (100Ω) resistor in series with the Bipolar Transistor Base usually squashes these parasitic induced oscillations.You might consider a Wilson Current Mirror instead of the Cascode.Best,

author: francogomme
author_rank: Newbie
timestamp: March 04, 2025, 04:02:33 pm
content: Thank youThe capacitors were added later, on the suggestion of ChatGPT. I asked how to stabilize the circuit, and it gave me these indications. I felt it was unnecessary to update LTspice because I'm experimenting with the circuit design. In the future, I will have to create a PCB with quality components. For now, I'm building a test circuit to see if the idea is good and works. I encountered these problems, and I don't understand their nature and I want to understand them better.And before you can say to me, yeah i know that chat gpt is a poor source for advice on circuit design and i also say that a desperate man might end up doing anything in life

author: francogomme
author_rank: Newbie
timestamp: March 04, 2025, 04:06:36 pm
content: For being sure about what i understood about your suggest "Adding a small value (100Ω) resistor in series with the Bipolar Transistor Base", i have to put a 100 ohm between B(Q1)-B(Q2) and C(Q1) ?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2025, 06:14:13 pm
content: Current mirrors are tricky with discrete parts, as there can the thermal effects, that are usually not included in the simulations. A usual countermeasure are emitter resistors.For high precision I would not use discrete BJTs - real world parts have limited matching and thermal effects complicate things. The more sensible way is to used a FET and OP-amp.

author: mawyatt
author_rank: Super Contributor
timestamp: March 04, 2025, 08:25:25 pm
content: Quote from: francogomme on March 04, 2025, 04:06:36 pmAdd a smaller resistor in series with each transistor base to see if this helps with oscillations.Best

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 04, 2025, 08:26:37 pm
content: Quote from: francogomme on March 04, 2025, 03:26:17 pmYou would need to give a scope picture of these oscillations.Such a circuit is critically dependent on the24V stability, as that defines the reference current.Quote from: francogomme on March 04, 2025, 03:26:17 pmIt is unusual to try to make a calibrator that uses 24V as the reference voltage.More common is to use a voltage reference and a power device, usually a MOSFET, as you can need up to 480mW for a 20mA test pointIf you want asourcing current reference, a high-side reference plus 2 resistors plus a PMOS transistor is usually good enough.The classic LM431/TL431 is a low side reference, where the Vref is relative to the negative rail.Suitablehigh side referencescould be Adjustable version of LM4041/LM4051/TL4051, or NJM2823 or NJM2825 (etc)eg for 1.233V ref, you pick a source resistor of 1.233/12.00m = 102.75 ohmsAddit: Highest precision high side Vref I can find are the TI LM4051AIM3-ADJ at 0.1% & TI TL4051AID/TL4051AQD (adj) at 0.1%

author: tggzzz
author_rank: Super Contributor
timestamp: March 04, 2025, 10:28:27 pm
content: Quote from: francogomme on March 04, 2025, 03:26:17 pmFor a discussion of a similar circuit and situation, seehttps://entertaininghacks.wordpress.com/2024/03/16/practical-traps-with-a-one-transistor-audio-amplifier-solderless-breadboards-and-oscilloscopes/It demonstrates and discusses "unexpected" effects, their causes, and some solutions.