Laser diode driver oscillating

Hi,
I came across a circuit which can be used to drive a laser diode in continuous mode (CW) or modulated (<=100Khz).
A BFR540 (NPN) transistor is used to drive the laser diode and a photodiode (inside the laser diode package) provides an output current based on the optical output power, which is used as a feedback to control the optical output power. The latter is set via the potentiometer at the base of the BFT93 (PNP) transistor. Modulation, when needed, is achieved by switching the MOD line between 3.3V and GND.

The circuit is shown in the attachment.

As shown, it works correctly. HOWEVER, when I change the BFT93 (PNP) to any other PNP transistors, the output (laser power) oscillates at high frequency (>1MHz). The reason I'm having to change the BFT93 is that this part is now obsolete and hard to get.
If I increase the value of the capacitor at the base of BFR540 the system becomes stable, but I cannot modulate it at 100 KHz anymore.

I believe that this oscillation will be due to the poles and zeros of the closed loop being inappropriate, but I'm not sure how to find the required information from the circuit.

I would appreciate if anyone could provide any advice/suggestions. I think I gave all the information I can about the circuit and its behavior, but if more information is required please let me know.

NOTE: I have tried replacing the BFT93 with transistors which have the closest parameters (i.e. hfe, Vce, power, etc), but I couldn't find any that matched the frequency (5GHz).

Where did you get this circuit?

I don't know how that is relevant to the problem at hand. Any suggestion/advice is appreciated.

I have used that circuit before and the transistors are actually pretty forgiving... the circuit is a self regulating throttle to optically limit the current and brightness of the laser. Double check the BEC of the substitute transistor you are using not all SOT-23 transistors have the same BEC pinout.

Beau Schwabe said:

I have used that circuit before and the transistors are actually pretty forgiving... the circuit is a self regulating throttle to optically limit the current and brightness of the laser. Double check the BEC of the substitute transistor you are using not all SOT-23 transistors have the same BEC pinout.

Click to expand...

Thanks for your reply.
My first thought was that the pinout of the replacement transistor was different, as you mention, but this was not the case. One replacement I tried was the MMBTH81 and as you can see in its data sheet the pinout is the same as the BFT93 (for SOT-23).
What do you mean when you say that the "transistors are actually pretty forgiving"?

Perhaps I should mention that I can get the laser to work correctly in continuos mode (replacing BFT93) if I increase the value of the capacitor at the base of BFR540. However, this prevents the laser from being modulated at high frequencies (around 100kHz), which is undesirable.

If you have any other suggestions I'm all ears.
Thanks

nfdc said:

I don't know how that is relevant to the problem at hand. Any suggestion/advice is appreciated.

Click to expand...

Because often people grab unreliable circuits from unreliable sources.

Using LT spice:

D1 is a LED with about the same 2 volts drop.
V2 is a voltage source of zero volts. It is a place where I can measure current. It functions as a wire.
F1 & V2 is the "measure current function" in the laser diode. The current from F1 is (0.003 * current in V2).
You can't see it but C1 1nF has a internal resistance of 330 ohms that speeds up Q1 and helps with overshoot. (C1 = 1nF and 330 ohms)
D3 is a diode to keep Q2 from saturating. This speeds up Q2!.
R4 is near zero ohms. (no effect at 0.05 but has effects around 5 ohms) It also effects overshoot and reduces the gain of the amplifier.
R2 sets the current.

Probably because I used some slow transistors the driver turns on too hard. The current gets too large for a short time and rings. Adding series resistance to C1 helps. Speeding up Q2 helps. (D3) Playing with C1, 1nF effects the amount of over shoot and ring. R4 = 5 ohms also effect the ring.

Have fun playing with it. See attached fine.