High Speed, High Current Op-amp

Question

I'm looking for high speed (slew rate), high current op-amps. I'm aware of the external typologies that can be added to op-amps to increase the output current capability, but the component count starts to become daunting for my application. I'm trying to design a circuit to produce up to a 1.5 Amp current pulse to an optical device. The pulse will vary in width from 100 ns to 1000 ns depending upon the application. Rise/Fall less than 10 ns. I've been googling for a bit, and it seems to be exclusive. You can get either the high current, or the high slew rate. Any options would be great.

D_Hersey · Answer

Bi-FET reefers to amplifiers that use both bipolar and field-effect transistors in the same circuit. Early on, it was often a primitive mosfet driving a bipolar Q. This yielded a composite device that had the high Z-in of the mosfet and the ruggedness of the bipolar. Connecting the drain of the mosfet to the collector of the bipolar was often possible, this made the mosfet current flow through the load as well as the bipolar. This kept things relatively efficient. In practice mosfet control terminals usually look like capacitors, which bipolars can be very good at slewing. Nowadays, since mosfets have gotten better, a mosfet is often the final. A bipolar in common-emmiter mode is an inverter, providing 180 degrees of phase. A mosfet in common source, driven by the switching bipolar also imposed 180 degrees of phase. So the overall amplifier would have 360 degrees, equal to zero degrees (with a little delay) of phase. The bipolar amplifier, on the left side, has an input resistor, typically, to convert the control potential into a current. If we attach a large resistor from the final (mosfet) output all the way to the (bipolar) input, when the power switch we are constructing starts to turn on, load current (due to Q2) will make it turn on harder. The on-to-off threshold has been biased away from the off-to-on threshold, thereby creating a Schmidt trigger. This output side would not be good for audio, unless we are doing class D. It makes for a very crisp pulse amp in terms of rise and fall times because those characteristics are not set by the driving pulse. In a sense this output stage 'cleans up after' the controlling pulse.

D_Hersey · Answer

An emmiter-follower totempole at the gate of a power mosfet will work as well as a gate driver, at about a tenth of the cost.

michaelkellett · Answer

This'll be quite hard - up to 1.5A in 10ns and nice and clean - means short wires between load and power device, very low impedance supplies. Check out this (Linear technology APP Note 133) www.linear.com/docs/40550 Jim Williams is only aiming for a much slower rise time and single quadrant drive but at higher current - note well the section "Layout Effects". About the simplest way to get close (which oddly I've just done for someone) is to uses a MOSFET with proper MOSFET driver and control the current by having some series resistance and an adjustable;e power supply. My circuit (in the picture) has 1.175R series resistance, 5 low ESR 22uF caps in parallel, and 10R in series with the MSOFET gate - rise time to 2A is about 35nS. I've got another one to build and test so if I get time I'll try without the 10R and take some scope pictures. It would probably work better with the MOSFET closer to the LEDs. I'd like to get the max current up to 5A. This board is at the end of 3M of cable (single twisted pair with two additional power cores) and uses LVDS TX and RX chips to get the control pulses up the wire. The power regulator at the source end of the cable can do about 4-14V. The load is 4 red LEDs in series (it's a strobe light but I can't tell you what its for.) If I needed to get current out of the load fast I'd add another MOSFET (P) to short it the load out (and worry about how to stop them both being on at once !). MK

High Speed, High Current Op-amp

Top Replies