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transistor selection

M

merco

Jan 1, 1970
0
hi,
I need some suggestions in complementary transistors selection.

I have to design a class AB amplifier (push pull) with IOut max = 1
App for a >=100Khz and <=170KHz sine wave signal.

Any ideas ?
or link to sites ?

Thanks
 
E

Eeyore

Jan 1, 1970
0
merco said:
hi,
I need some suggestions in complementary transistors selection.

I have to design a class AB amplifier (push pull) with IOut max = 1
App for a >=100Khz and <=170KHz sine wave signal.

What load impedance ? And what volts ?

Most of the modern audio fast output transistors from the likes of Toshiba will
do that fine ( fT >= 20 MHz ).

Graham
 
J

John Larkin

Jan 1, 1970
0
hi,
I need some suggestions in complementary transistors selection.

I have to design a class AB amplifier (push pull) with IOut max = 1
App for a >=100Khz and <=170KHz sine wave signal.

Any ideas ?
or link to sites ?

Thanks

Why do you "have to"? What's the application? What's the load?

John
 
E

Eeyore

Jan 1, 1970
0
merco said:
4 ohm / 7Vpp...

Wow. I'm not used to looking at anything that low power normally.

Are you looking for an SMT part ?

Graham
 
R

Rich Grise

Jan 1, 1970
0
Why do you "have to"? What's the application? What's the load?

Probably because if he didn't he wouldn't get a good grade. ;-)

Cheers!
Rich
 
T

Tom Bruhns

Jan 1, 1970
0
4 ohm / 7Vpp...

That's only about a watt and a half. What power supply voltage will
you run from? It should be easy to do with a single supply, capacitor
coupled output; a 4.7uF polypropylene cap would have around a third of
an ohm reactance at 100kHz, and very low loss. The 44H11/45H11 pair
should do well at this current and power level, and have ample voltage
ratings.

You might even find that an integrated "audio" power amplifier would
do fine, at a cost savings if you consider the sheer number of parts
needed in a discrete design and the cost of putting them on a board.

Also, over that small frequency range and low power, you could easily
transformer-couple the output, and use a pair of NPNs push-pull. It's
likely that one of the small MiniCircuits "RF" transformers would work
fine; at 4 ohms instead of 50, you'd get response to lower frequency,
and you'll find that even at 50 ohms they have quite a few models that
cover to below 100kHz. You could easily run from a single 3.3V or 5V
supply, or pick a higher impedance ratio to run nicely from 12V or
other voltage. For example, with a 12V supply, you can deliver 7Vp-p
with a 36:1 impedance ratio (3:1 turns ratio from _each_half_ of the
primary to the secondary). Lest you think that those small
transformers couldn't handle the power, I can say I just tested one
(TT1-6) at about half a watt, which is all my RF signal generator will
do easily, and it behaved well at 100kHz. I can't guarantee that that
particular one would handle 1.5 watts, but I'd expect some in their
lineup would.

Cheers,
Tom
 
D

David Starr

Jan 1, 1970
0
merco said:
hi,
I need some suggestions in complementary transistors selection.

I have to design a class AB amplifier (push pull) with IOut max = 1
App for a >=100Khz and <=170KHz sine wave signal.

Any ideas ?
or link to sites ?

Thanks

First compute the heat developed in the transistor. That's equal to Vce
* Ic. Assume VCE is a sine wave and Ic is a sine wave 180 out of phase.
Either evaluate that function and allow for the 180 cutoff (transistor
is off for half the cycle. Or dig out somethine like the Radio
Amateur's Handbook and find a formula for power in class AB amplifiers.
Given the power requirements select your PNP transistor. Holes have
lower mobility than electrons so all things being equal the PNP
transistor will have lower Ft than a corresponding NPN. Go for the best
Ft you can find in a device that can handle to power.
Then use 0ja (thermal resistance case to ambient) to compute
junction temp on your selected device, with the heatsinks you may or may
not need at the ambient temp you expect. I feel that junction temps
should be less than 125C, preferably 100C, but others have run devices
as hot as 200C. If you cannot show a safe junction temp, you need a
bigger package or a bigger heat sink or both.
Given a good PNP device, scan the selector guides for a reasonable
match in NPN. You want the same case style, the same Ft, the same
breakdown voltage and collector base capacitance. Don't worry about hfe
or beta.
Finally make sure the devices you have selected are really
available. Can you get them from Digikey? Are they on the shelf?

David Starr
 
T

Tom Bruhns

Jan 1, 1970
0
First compute the heat developed in the transistor. That's equal to Vce
* Ic. Assume VCE is a sine wave and Ic is a sine wave 180 out of phase.
Either evaluate that function and allow for the 180 cutoff (transistor
is off for half the cycle. Or dig out somethine like the Radio
Amateur's Handbook and find a formula for power in class AB amplifiers.
Given the power requirements select your PNP transistor. Holes have
lower mobility than electrons so all things being equal the PNP
transistor will have lower Ft than a corresponding NPN. Go for the best
Ft you can find in a device that can handle to power.
Then use 0ja (thermal resistance case to ambient) to compute
junction temp on your selected device, with the heatsinks you may or may
not need at the ambient temp you expect. I feel that junction temps
should be less than 125C, preferably 100C, but others have run devices
as hot as 200C. If you cannot show a safe junction temp, you need a
bigger package or a bigger heat sink or both.
Given a good PNP device, scan the selector guides for a reasonable
match in NPN. You want the same case style, the same Ft, the same
breakdown voltage and collector base capacitance. Don't worry about hfe
or beta.
Finally make sure the devices you have selected are really
available. Can you get them from Digikey? Are they on the shelf?

David Starr


I assume by "best" Ft, you mean one adequate for the job, not the
highest one. Picking one too high will just invite stability
problems. Certainly I can get parts that have very high Ft's that
will handle the power; doing so seems a waste and an invitation to
trouble.

Cheers,
Tom
 
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