Current mirror use

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Mosaic

Well-Known Member
Hi All:
Is it possible to design an amplifier using multiple current mirrored 2n3906 (rather than a multistage amp) to deliver a higher wattage amplifier output for a given Vcc? 50 ohm output load, perhaps 12 to 36Vcc supply.

Would there be linearity issues?

I haven't done an LTSPICE sim yet.

MikeMl

Well-Known Member
Hi All:
Is it possible to design an amplifier using multiple current mirrored 2n3906 (rather than a multistage amp) to deliver a higher wattage amplifier output for a given Vcc? 50 ohm output load, perhaps 12 to 36Vcc supply.

Would there be linearity issues?

I haven't done an LTSPICE sim yet.
What kind of signal. If audio or RF, then without using a transformer, the highest power you can get into a 50Ω load is E^2/R, where E is the rms value at the output of the amp, and R is 50.

The rms value when starting from Vcc = 12 is only ~4Vrms, so P = ~16/50 = 0.3W

The rms value when starting from Vcc = 36 is only ~12Vrms, so P = ~144/50 = 2.9W

AnalogKid

Well-Known Member
For a current mirror to work, the hfe of the transistors must be very tightly matched, and track over temperature and aging. This is relatively easy when all Q's are on a single, very small silicon die, but much more difficult with discrete components. There are more complex current mirror circuits that work to offset or cancel out some of these effects, but the net result still is far from what goes on inside ICs. Why do you want to take this approach? What advantage do you think it might have over conventional circuits?

ak

alec_t

Well-Known Member
I agree with AK. Can't you use a standard opamp with a push-pull output buffer added, or else use a power opamp such as the OPA541?

Mosaic

Well-Known Member
Thinking of replacing an RF power module with lower cost parts. I believe with a class C resonant tuned approach the Vout can be higher than the V in for more wattage.

Nigel Goodwin

Super Moderator
Thinking of replacing an RF power module with lower cost parts. I believe with a class C resonant tuned approach the Vout can be higher than the V in for more wattage.
Yes it can, but I fail to see the reasoning for current mirrors?.

Mosaic

I was looking at RF amp modules and they cost some $$. I wondered if spreading the load over jelly bean transistors with sufficient Ft would be an alternative that's more cost effective (price breaks etc.) where PCB space is not an issue. Nigel Goodwin Super Moderator Most Helpful Member I was looking at RF amp modules and they cost some$$\$. I wondered if spreading the load over jelly bean transistors with sufficient Ft would be an alternative that's more cost effective (price breaks etc.) where PCB space is not an issue.
Again, what have current mirrors got to do with it?.

You can certainly parallel transistors for RF output, it's done all the time - but it's probably not practical for man than a few, and makes the circuit much more complicated and more critical to build.

ci139

Active Member
what's the frequency ... ... coz if it goes to antenna line it's not the place to optimize
becides mmbth11 there're some interesting schematics at the end while mmbt3904 has not

Mosaic

Well-Known Member
Looking at the FM band, community/crime watch radio.

ci139

Active Member
below 160MHz does not necessarily require special transistors instead of 2N3906 however using better ones won't hurt coz operating bjt near it's limits requires a lot of such in parallel for P.gain - P.drive to make difference (the spice simulation - i don't build HF stuff too often - shows real effect on gain - more - using 2x in parallel - less - using 3x in parallel ... for more x you start loosing [email protected])

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