How to think about op-amps and highly capacitive loads? - Page 1
Forum section: electronics / beginners

author: ralusp
author_rank: Newbie
timestamp: June 29, 2025, 01:37:53 am
content: I'm planning to use an OPA551 op-amp in a non-inverting configuration (gain 11.5) to drive two ultrasonic transducers (328ST160) in parallel on a PCB, and am unsure how to extrapolate certain datasheet values and plots.Each transducer has an impedance of 900 ohms and capacitance of 2.4 nF at 1kHz. They'll be driven with a signal at 32.8 kHz (the resonant frequency), so I'm not sure how to reconcile the datasheet capacitance specified at 1 kHz. Then, in the OPA551 datasheet, Figure 19 shows small-signal overshoot vs. capacitive load for G=1, indicating ~40% overshoot at 450 pF. I believe this scales up linearly with gain, but how do I assess what level of overshoot is acceptable?I'm also considering placing a 10 ohms isolation resistor between the op-amp output pin and the AC coupling capacitor (just after the feedback loop junction), and another 10 ohms damping resistor after the capacitor. I understand this helps with stability, but I'm unsure how to quantify the improvement or optimize resistor values.I have prototyped the circuit on a breadboard and it appears stable, but I'd like to be able to more rigorously assess the stability margins to ensure I'm not operating too close to the edge, and to consider whether a third transducer could be safely added later. I'd greatly appreciate any advice! I've also attached a draft of my circuit.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 29, 2025, 09:05:51 am
content: See attached on how to and the details of managing capacitive loads.Its always worth simulating your circuit eg with QSPICE as the first step in your sanity check.There are opamps that will drive capacitive loads directly. eg OPA392, LM8261, LM8272, LT1360 and many more.Consider using ouptut transformers rather than HV opamps and all the psu madness that goes with them.www.coilcraft.com/en-us/products/transformers/power-transformers/ultrasonic-sensing/

author: ralusp
author_rank: Newbie
timestamp: June 29, 2025, 10:42:18 pm
content: Thanks for the suggestions and resources! I don't have much experience in this area and am not really clear on the tradeoffs of using an output transformer vs. opamp, but I'll do some research there. Ultimately, I'm looking for an AC-coupled 36Vpp output, so the suggested opamps don't quite reach that voltage. I'll build up a SPICE simulation as my next step here..

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 01, 2025, 12:46:18 pm
content: One thing I was taught a long time ago is that opamps with rail-to-rail outputs are much more sensitive to capacitive loading. This is because their output stages use common source/emitter (high output impedance), rather than source/emitter followers (low impedance). The higher output impedance means the load capacitance introduces a much lower corner frequency in the open loop transfer function.The OPA551 doesn't clearly state how its output stage is built, but it's not rail-to-rail so it's likely source/emitter follower. The OPA551 datasheet actually has a section on capacitive loads. It even suggests a circuit for capacitive loads up to 10nF:I'll admit I'm a little puzzled as to why this circuit works. The capacitors only introduce overall phase lag in the feedback path, not what you'd typically want if stability is a concern. But somehow this creates phase boost in the loop transfer function... I must be forgetting something very basic, or it's just too early in the morning.

author: jwet
author_rank: Frequent Contributor
timestamp: July 01, 2025, 01:02:01 pm
content: This is slightly more interesting than just driving a capacitive load. Because the transducer is resonant, the complex impedances will cancel out and the driving impedance will be real. The low frequency capacitance will still be there but if you don't excite it (drive any signals to it at low F's), you won't see it. What you'll see is an almost completely real impedance. Your amp shouldn't produce any low frequencies. Narrow band is your friend. Look at coupling capacitors and inadvertent gain at low F's (that 1 uF coupling looks suspicious). I would Spice it. It would be nice if you could model the transducers as being resonant. A good approximation would be to assume that low F C is there and at 32.8KHz, it resonates with a mythical L, you can calculate this value. I say mythical because the mechanism of piezo resonance is different but the equivalent effect on the circuit should be close.Have fun.