Electronics > Projects, Designs, and Technical Stuff
HV amplifier: putting a common-source (emitter) amplifier on opamp outputs
Whales:
Every negative-feedback amplifier I have seen uses uses emitter followers (or source followers) on the outputs. Why is this so?
I think it might be easier to keep a follower stable in a feedback loop (because the transfer function between gate/base and output is more continuous and more spread out).
I also know that you can't make a BJT push/pull in a common emitter configuration: the two bases feed into each other. You have to separate with previous stages OR use mosfets OR make it class A (one transistor) instead.
Context
I have a friend with a high-voltage piezo positioning device (~1uF claimed) that he wants to drive with 0-150V from DC to 10's of KHz.
At the moment I'm working on recreating a design that a previous team already claims works -- a big discrete HV opamp, all bipolar with a long tail pair on input and several stages of amplification to get to the output push/pull pair. There are also several off-the-shelf products (mainly made by Apex) that would appear to meet many of our specs.
ie I know there's off the shelf stuff, this topic is for fun & education.
My whacky idea
Class A common-source amplifier on the output of a low-voltage opamp. Performs the voltage shifting and heavy work for me:
I've made it mostly stable through the addition of the resistor between opamp inputs. Source.
Seems very slew-rate limited, can't operate much above 10Khz. This makes sense given the 100 ohm upper resistor + 1uF load.
You may notice the opamp inputs seem backwards. This is because the common-source mosfet acts as an inverter.
Other physical notes:
* High-wattage pullup resistor is probably going to be a collection of bulbs, kept under-powered so they don't change R too much. Who cares about efficiency :D
* I'm aware of SOA and have been reading the EEVBlog HV amp mega-topic. The IRF640 might not be the best option here, it's just something my SPICE software has, and I plan to use several in parallel if necessary.
* Source resistor is only approximate, it's being used to keep the sim happy. End result will have some form of source and/or drain resistors (esp if I end up paralleling fets).
Simulation note: I'm using QUCS and it really doesn't like doing transient analysis on this circuit (gets stuck in infinite loops or complains of jacobians). Several workarounds:
* Source resistor (under mosfet): increasing this sometimes nukes the jacobians.
* Small hinter circuitry (inside dotted box): I think this helped avoid infinite loops in analysis, not sure now. I suspected that the solver was getting confused by the fact that changing the gate voltage on the mosfet didn't always change the voltage at the output, ie there was no gradient to descend.
* I needed to set the 'minimum step size' in QUCS' transient simulation very high. default: 1e-16, my setting: 1e-7. This prevents the infinite loops (most of the time).
David Hess:
Some designs operate that way however the problem is that the common emitter/source stage adds voltage gain which depends both on operating point and the load impedance and this makes frequency compensation difficult. The added voltage gain requires the open loop gain of the operational amplifier or gain-bandwidth product to be reduced; the resistor added between the inputs has that effect.
Two ways to improve the situation include:
1. Add emitter/source degeneration to better control the voltage gain. This turns the transistor into a more linear voltage to current converter.
2. Add shunt feedback to the transistor stage to make its voltage gain fixed. This is common with voltage boosted operational amplifier circuits.
TurboTom:
In order to reduce power consumption, you may use a diode-dropper driven bipolar active pull-up. This will introduce some distortion upon cross-over (i.e. a slew rate change) but depending on the requirements, these problems usually are tolerable ones. I designed a scope clock with the high voltage deflection drivers arranged that way since I had to keep power consumption as low as possible. See here for the schematic to get an idea. The schematic shown settles within about 1µs after a 50V step. Quiescent current is less than 1mA for each channel.
NiHaoMike:
A circuit similar to that (small flyback converter + discharge resistor switched in with a MOSFET) is used for driving piezoelectric expansion valves in some variable speed HVAC systems, but that only operates with a bandwidth of a few Hz at most. For something that has to operate into a few tens of kHz, the most obvious solution is to adapt an audio amplifier design.
Marco:
--- Quote from: Whales on December 16, 2019, 11:25:16 pm ---Simulation note: I'm using QUCS and it really doesn't like doing transient analysis on this circuit
--- End quote ---
Try Simetrix, only downside is that more complex opamp models quickly take you over the node limit of the free version.
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