I'm trying to make a current buffer for high currents, using an LM324 and BUZ70. This is the circuit:

![schematic](https://i.stack.imgur.com/8GMxk.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2f8GMxk.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

I've noticed that this circuit starts to have a little oscillation (at least 100 times more the Vs frequency) overlapped to the gate voltage if a gate resistance of around 10Kohm is used. **Removing this resistance, will remove the oscillation.** 

Just for completeness, there is also an evident miller effect, in which the initial ramp before the miller plateau is enlarged if a gate resistor of 10K is used, and this should be coherent.

The oscillation disappears also if the led is fed at 5V. 

If Vs is a DC voltage, the oscillation takes place too, showing that it is NOT a transient ringing.

It is a breadboarded circuit, but in any case, I think that I have a very little phase margin, but this circuits should have a very large bandwidth.

Any opinion?

**edit:** when the LED is turned on, fraction of this oscillation is present in V\_Power. I may solve this by putting a cap here and there and not using an high gate resistance, but I'd prefer to understand why, if it is possible.

**edit1:** as stated in comments, I need a compesation if the source resistance is less than 50 ohm. I don't understand, because modelling the system using this circuit:

![schematic](https://i.stack.imgur.com/h7pt1.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fh7pt1.png)

I obtain

$$V\_{out}= \frac{R\_s G\_m}{1+R\_s G\_m} \frac{1-\frac{s}{z\_1}}{1-\frac{s}{p\_1}} V\_{oa} = k V\_{oa}$$

with $$z\_{1} = -\frac{g\_m}{C\_{gs}}$$ $$p\_{1} = -\frac{1}{C\_{gs}\frac{R\_s}{1+g\_m R\_s}}$$

and this does not affect the stability of the opamp, which is an LM324 with a limited band. The Cgs used is the provided Ciss, the gm = 2 (because of the limited current used now, see datasheet of BUZ70).

In fact, this MOSFET stage have the Vout/Voa with a bode of:

[![Matlab simulation](https://i.stack.imgur.com/yKD7o.jpg)](https://i.stack.imgur.com/yKD7o.jpg)

Therefore I can consider the system with the OPAMP asa voltage follower. Why I need to add compensation with low Rs values?