# Constant DC Load Stability and Bandwidth slewrate

For this circuit, how do I know if the rise edge was limited by slew rate or bandwidth? What's the effect of R2 and C2 on bandwidth?

-3db of this circuit in simulation is 649 kHz, I know in this circuit, R1 and gate capacitance, R2//C2 , will affect bandwidth, but it doesn't add up with 2πfc formula.

NE5534 has 10 MHz bandwidth, and 13 V/µs slewrate.

I was trying to decrease C2 and R2, rising time shortened, but it comes with stability issue, with overshoot and gain peaking, how can I break the loop in order to see its bode plot? Also any idea on how to increase raising time will be appreciated.

Frequency phase analysis

Without R1, R2 and C2, the circuit will be unstable because T1 has gate capacitance and the bare op-amp has a high output impedance (500-ish ohms sticks in my head as typical for most op-amps, but it's hard to get solid numbers). That R-C circuit is inside the feedback loop, and it causes enough phase shift to make the collection oscillate.

R1 isolates the op-amp output somewhat from the gate capacitance. C2 and R2 decrease the overall bandwidth of the op-amp to roughly 1/(R2*C2) radians/second. This compensates the circuit for the capacitive load.

I'm not sure of the best way to break the loop, but if you just connect a resistor to the - input of the op-amp (10k or so) and drive it with a sinusoid, you should be able to monitor the output to see if it looks well behaved. You'll basically be inferring the open-loop characteristics from the closed.

For a faster rise time, probably the best thing you can do is to find an op-amp that has a lower output impedance. That's not a data-sheet number; I'd look for devices that are designed to drive a lot of current, or are designed to drive capacitive loads.

If you want to get a bit out on a limb, choose a nice fast, small BJT (with low base capacitance!) and connect it as an emitter follower ahead of R1. You may have to add a resistor to ground to help turn the FET off (and to heat up your board's surroundings -- you didn't want your circuit to be TOO low power, right?).

Note that this isn't a place where I'd trust simulation much -- you'll want to prototype this with the specific op-amp you're going to use. If you can get the attention of an applications engineer you may want to work with them -- sometimes those guys have the sort of inside information you need.

• The impedance you mentioned is open loop output impedance which forms RC with C2. IRFZ44n has Ciss 1470p, if I change it to IRF530 which has Ciss 670p. I can lower C2 and R2 with same stability, and the circuit speed up a bit. – SpringRoll Wang Oct 11 '18 at 18:59
• @SpringRollWang: Yup. My emitter-follower suggestion may get you an order of magnitude of improvement, but it'd be much more work. – TimWescott Oct 12 '18 at 20:20