High-frequency power amplifier circuit

[jackmorrison]

I've gone a little outside my area of expertise in a section of a circuit I'm working on, and I'd be grateful for some advice. I need to boost the current drive of a voltage source to power an array of LEDs. Requirements for the circuit are:

Input: range as needed, nominally 1V to 3V, frequency DC to 5MHz; high impedance (direct from DAC or waveform generator)

Power: single-ended supply, prefer 6.5 to 12V

Output: about same voltage range as input (so gain of 0.5 to 1), at least 500mA, prefer 1A (continuous). Load is 0-64 high-brightness LEDs via analog switches with switch capacitance up to 130pF each, LED capacitance unknown but maybe guess 10pF, so 10nF total

Size: ideally no more than 10x10mm for two channels

My initial design uses OPA2673 in voltage follower configuration: input to +IN, -IN tied to OUT; single-supply (-VS=ground, +VS=10V). This works for a little while, then the ($4) IC heats up and dies - even with no LED load connected. Initially I had the "GND" pin on the IC tied to ground, but now I think that's wrong. Also I notice the datasheet recommends 511ohm feedback resistor even in a voltage follower configuration. This part is only available in a QFN package which is a little tough to experiment with (although there are lower-power relatives available in SOIC).

Now I'm wondering if a simple NPN transistor circuit would be better. I threw together a test with a 2N2222 I had laying around, and it seems to work but a little slow.

So my questions are:
- what's the disadvantage of a single BJT voltage follower compared to an op-amp?
- any idea why the OPA2673 is not happy in this role?
- what's the right way to meet the above requirements?

Thanks for any help.

 

[kripacharya]

Thanks for the heaps of data & specs. I'm sure the forum (or myself) can provide useful answers.

However a few questions before I do that --

Why does your signal input need to be DC to 5MHz ? Since its driving LEDs, what is the actual application ?
Whats the purpose of the additional analog switch ?
How many LEDs are you driving from a single source ? Presumably they are in parallel ?

 

[keith1200rs]

With the inverting input shorted to output the opamp is probably oscillating. This will make it get warm but shouldn't kill it as it has thermal protection. You would need to check with a high bandwidth oscilloscope as it will be around 400MHz.

I would probably either use a fast opamp to drive a MOSFET or a MOSFET driver IC driving a MOSFET. Another option is to use a MOSFET driver IC directly (no MOSFET) as there are plenty that can drive more than 1A.

Keith

 

[jiripolivka]

I do not see the exact purpose of your device. To INDICATE signals from 1 V p-p to 3V p-p, for DC to 5 MHz, I used a fast comparator like MAX999. Adjusting reference input voltage can start triggering from inputs above ~ 20 mV. You can use more comparators with several reference levels to make something like VU meter with LED strings.

 

[jackmorrison]

Why does your signal input need to be DC to 5MHz ? Since its driving LEDs, what is the actual application ?

It's an unusual one: a 64x64-dot matrix display for neuropsychology research in perception. DC to set a fixed intensity, and up to 5MHz to study modulation and ramp effects.

Whats the purpose of the additional analog switch ?

Each LED can be switched between a "foreground" and "background" intensity level (hence the two amplifier channels), again at sub-microsecond timing.

How many LEDs are you driving from a single source ? Presumably they are in parallel ?

The design has one board for each 8x8 section of the matrix, so 64 boards make up the full display. One master power source and the two signal sources feed all the boards in parallel. Each board drives its 64 LEDs in parallel. There can't be any multiplexing or raster scanning that an LED matrix would normally have. And since they're in parallel with the number of enabled LEDs switched rapidly, I have to use voltage control for intensity rather than the preferred current control.

- - - Updated - - -

With the inverting input shorted to output the opamp is probably oscillating. This will make it get warm but shouldn't kill it as it has thermal protection. You would need to check with a high bandwidth oscilloscope as it will be around 400MHz.

I did see about 20mV p-p oscillation with no LEDs connected, forgot to note the frequency.

I would probably either use a fast opamp to drive a MOSFET or a MOSFET driver IC driving a MOSFET. Another option is to use a MOSFET driver IC directly (no MOSFET) as there are plenty that can drive more than 1A.

Interesting, I'll look into that. I've used MOSFETs as on/off switches before, but not for linear operation. Would that be more efficient than the BJT?

Thanks.

 

[keith1200rs]

Interesting, I'll look into that. I've used MOSFETs as on/off switches before, but not for linear operation. Would that be more efficient than the BJT?

You can include either bipolar or MOSFET emitter/source followers within the opamp feedback loop.

Keith

 

[jackmorrison]

You can include either bipolar or MOSFET emitter/source followers within the opamp feedback loop.

Keith

That would be to reduce the input impedance vs. driving a transistor directly? I guess that might be needed. So I could use a lower-power (but still high-speed) op-amp. But other than running away from the OPA2673 that's giving me trouble, why add the complication of an additional stage?

 

[jiripolivka]

I would still recommend using the fast comparators. Their sensitivity starts from ~20 mV and levels can be set by reference voltage (even variable is possible). MAX999 can operate at >10 MHz. This can simplify your front end and then you can play with the wonderful display.

 

[jackmorrison]

I would still recommend using the fast comparators.

I don't see how a comparator would help here - this isn't a digital indicator; I need to vary the brightness of each LED.

 

[jiripolivka]

I don't see how a comparator would help here - this isn't a digital indicator; I need to vary the brightness of each LED.

Using two or three comparators with staged reference voltages, you can vary LED brightness in two=three stages. I do not think your eye can distinguish more.

If you prefer to play with fast opamps, good luck. I wanted to help.