Hello, BJT experts.
I would like some suggestions for a good through-hole NPN/PNP pair to use for a push-pull amplifier, with max Vce of about 33V, max Ic of about 200mA, and max Pd of about 1W (AVG Pd is about 600mW). Bandwidth of the amplifier is about 1 MHz. (But also see "possible gotcha", farther below).
I would LIKE to not have to use heatsinks, if possible, if TO-220 devices are used.
Board space will be limited. TO-220 or TO-39/TO-5 or smaller should fit. However, there probably wouldn't be enough space left for a heatsink that extends horizontally, much, in any direction.
At lower power, I was using TO-92 devices (2N4401/4403), with neat little Aavid push-on heatsinks with a slotted vertical fin, Mouser.com # 532-575200B00. For TO-39/TO-5, they also have two different vertical-mount-type heatsinks that look like they would fit on the board, with thermal resistances of 35degC/W and 40degC/W. If I could use TO-220-cased (or similar) devices, I was hoping to be able to avoid using a heatsink, altogether (although some of the smaller "hat section" ones that sit up pretty high on the device might fit).
And, even though the Avg Pd is only about 600mW, the TO-92
2N4401/4403-type devices "apparently" can't be used, even WITH a heatsink, since, on the breadboard, they almost always immediately departed their cases at power-up, even when the avg Pd was only about 500mW and heatsinks were on (And that was before I added boost caps from output to split bias resistor pairs' junctions, which raised Pdavg to about 600mW).In case anyone needs more details about the amp:
It has +/-17.5v supplies, NPN and PNP with emitters tied together through 1 Ohm resistors to the output, collectors to rails, two 806 Ohm resistors in series from rail to base for each transistor, with 47uF (could be larger) from each 806 Ohm resistor pair's junction to the output, 1N4148 diodes from input to each base (cathodes toward PNP base, mounted in thermal contact with their respective transistors), and two 1N4148 diodes in (anti)parallel from input to output.
The amp is fed by a high-speed opamp (1/2 LT1364) and is inside two of its feedback loops, and can act as either a current source or a voltage source. The load is fed through a 100 Ohm 2.25W resistor (and a 3 Ohm resistor that is part of the opamp feedback loop), when in current-source mode. And there is another 100Ohm/2.25W resistor that can be switched in, from amp output (actually, from between the 3 Ohm and 100 Ohm 2.25W resistors) to GND, for voltage-source mode.
The opamp and feedback parts of the circuit are configured thusly:
Same supplies, two 4.99k R's in series from opamp's + input to gnd, two
4.99k R's in series from opamp's - input to source, 300R and 3R in series from push-pull's output to opamp's - input, 300R and 3R in series from push-pull's output to opamp's + input (Overall output is taken from between THOSE two 300R and 3R resistors.). There is also a 3.3pF cap from opamp's output to opamp's - input. And there is a large resistance (about 3.85Meg) from the negative rail to the junction of the two 4.99k R's that go from opamp's + input to gnd, to try to zero the overall output's offset.The overall output is taken from between the 300R and 3R that go from push-pull's output to opamp's + input, with 100 Ohms/2.25W always connected in series (from the overall output) with the load. And there's another 100 Ohms/2.25W to GND, with a switch that can optionally connect it to just before the first 100 Ohms/2.25W R, for selecting voltage-source mode instead of current-source mode.
This whole thing might be used to provide base/gate staircase drive signals to a device-under-test, for my curve tracer, with maximum total step voltage and current of 15v and 150 mA (at 1V or 10mA per step), and minimums of 3mV and 30uA (at 200uV or 2uA per step). The input attenuator's output voltage range would be the same (200uV to 1V per step, in 12 selectable "1-2-5" ranges). The overall output-amp voltage gain is 1. And there are buffer opamps between the attenuator and the output-amp's input, one inverting and one non-inverting, with a switch to select either positive or negative staircase output. The steps only transistion to another step at a maximum rate of 22kHz. But the rise-(or fall-)time between steps might be as low as about 1 uS.
POSSIBLE "GOTCHA":
One thing that I am worried about is trying to get the very-low-level (200uV steps) signals through the push-pull stage. It works great in Spice. But, on the breadboard, it sometimes looks like it can't handle anything that's too close to zero. i.e. I think it might sometimes have no output instead of very small output. (I just started the breadboard testing and have to admit that I haven't tested it much with low-level inputs, yet.)
I do realize that there is a "deadzone" problem with push-pull-type topologies. But I guess I DON'T really know how MUCH of a problem that is, or if it's possible (and practical) to "balance" a push-pull stage well-enough to pass a 200uV signal.
So I guess I'm also wondering if I should just "chuck it" and go with some sort of power opamp type of device, instead of the push-pull BJT stage. Essentially, all I really need is to have more current available than a single opamp can provide. I could probably use even more than 150mA, too, for testing devices that need more base current. Maybe 500mA max would be a better goal?
Thanks for any and all suggestions, comments, etc, positive or negative.
Regards,
Tom Gootee