Help selecting a 1A BJT for 200KHz switching?

[Lightium]

I need help selecting a BJT that's rated for around 1 amp collector current and can handle 200KHz switching. VCE should be greater then 20V.

I ask because as I increase the frequency on my oscillator section the driver sections output current reduces.

 

[Lightium]

Well I went through the specs of all the transistors and they should work as I suspect, so the problem is my design I guess.

 

[Nigel Goodwin]

200KHz is only a very low frequency, any transistor should be perfectly fine there.

 

[Lightium]

The only thing left to do was to double the supply voltage and it almost works.

 

[Lightium]

I need to be sure I selected the right Re and Ce of my common emitters.

Here's what used:

IE = (VCC - VBE) / (RE + Rth)

where:
VCC is the collector supply voltage
VBE is the base-emitter junction voltage (around 0.7V for silicon transistors)
Rth is the Thevenin equivalent resistance seen from the base (including biasing resistors)

 

[Nigel Goodwin]

Post the full schematic, so we can see what you're doing.

 

[Lightium]

I worked on this all day to get the output to acceptable levels, but I'd still like to increase the frequency and get the same output with 12 volts.

 

[rjenkinsgb]

I'd have thought C6 was redundant & C7 should be removed or across R11???
They are both attenuating the signal at high frequencies.

ps. Add series base resistors on Q3 and Q4.

 

[Lightium]

I'll give your suggestion a try in the morning, I am sooooo sleeeeepy.

 

[Lightium]

If I use base resistors on Q3 and Q4 isn't that gonna reduce current on the base's and reduce gain of the transistors?

 

[rjenkinsgb]

As it is, there are two different current paths that short part of the supply...

When Q1 switches on, it will pull the oscillator supply down via Q3 base-emitter junction.

When Q3 switches on, it shorts the supply via Q4+Q5 B-E junctions.

Nothing should ever short the supply!

 

[Lightium]

As far as I know, a short is an unintended path of electricity or larger then intended current flows.

R6 was chosen to limit the currents I believe you are referring to.

 

[Nigel Goodwin]

As far as I know, a short is an unintended path of electricity or larger then intended current flows.

R6 was chosen to limit the currents I believe you are referring to.

There is much wrong with that circuit, R6 shouldn't be there, and certainly won't be helping your problems - remove that and replace it with a piece of wire, and add a resistor feeding the base of Q3 (1K?). Plus remove the three spurious capacitors, C5, C6 and C7 - all those are doing is stopping your circuit working, I've no idea why you've just added random capacitors?.

Also Q1 and Q2 are likely to die from Vbe breakdown, as the supply voltage is too high - add diodes (1N4148 etc.) between base and emitter of each transistor, cathodes to bases, to prevent this.

I'm pretty dubious about the need for Q4 as well, and it could probably be replaced by a resistor - but with the suggested changes at least it has a chance to start to work.

I presume you haven't built this?, and are just playing with a simulator?.

 

[schmitt trigger]

I was writing something similar to what Nigel mentioned.
The circuit is flawed in so many aspects, that it would be FAR BETTER if the OP describes exactly what he is attempting to accomplish, rather than blindly attempting to make a flawed circuit to work, which may or may not solve his actual problem.

This is known as the XY Problem, and there is plenty of information on the web.

Home - The XY Problem

xyproblem.info

Now, if all he wants is to play with the simulator, he is most welcome to do so.

 

[rjenkinsgb]

As far as I know, a short is an unintended path of electricity or larger then intended current flows.

R6 was chosen to limit the currents I believe you are referring to.

And, if everything fed by R6 is shorted out, the main oscillator stops!
I'd call that "Unintended"! ???


Post the sim file as well as the schematic?

 

[Lightium]

I am happy with the way it is for now, but I wish I could get the same performance with a 12V supply.

 

[Nigel Goodwin]

I am happy with the way it is for now, but I wish I could get the same performance with a 12V supply.

Have you done the urgent changes we've suggested?, it will be useless without them - the asc file above is of no use to me, post a schematic of the updated design, so we can check it.

 

[Lightium]

Here you go.

 

[Lightium]

I've made changes, but it's not working as it was before. I honestly don't see how Q3 can power Q5.

 

[Nigel Goodwin]

I've made changes, but it's not working as it was before. I honestly don't see how Q3 can power Q5.

When Q1 collector goes LOW that pulls current through R6 and the base of Q3, this turns Q3 hard ON, and the collector goes to 12V or so, the 12V goes through R7 to the base of Q5, turning that ON. R7 at 6809 ohms will give about 17mA base current of Q5, lower R7 if you want more current.

D1 and D2 are good, R5 isn't needed (and can only mess things up), replace it by a piece of wire, also R10 doesn't need to be as low as 470 ohm, stick 4.7K in it's place.

I would also suggest R1 and R2 are rather low at 180, for no purpose, but if you've got your frequency where you want it, then it's probably easier to leave them be?.