I am planning to buy a few transistors for an amp I am making. The same transistors being manufactured by different manufacturers and that is why each one of them has a different hfe value. The hfe values range from 40 to 250.
My question is that.....is it okay if i decide to go with 40 hfe than 250. Devices with 250 hfe doubleible the price.
Does hfe matter a lot ? Does a low hfe value affect the performance of the circuit?
The transistor in question is BD 140.
Thanks
My question is that.....is it okay if i decide to go with 40 hfe than 250. Devices with 250 hfe doubleible the price.
Does hfe matter a lot ? Does a low hfe value affect the performance of the circuit?
The transistor in question is BD 140.
Thanks
What is the design? How much drive current is available and how much current are you expecting to deliver from the available drive current?
If you use an Hfe of 40 and have 5mA available, you will get 200mA. If you have an Hfe of 250 you will get 1.25Amps. The choice is yours.
If you use an Hfe of 40 and have 5mA available, you will get 200mA. If you have an Hfe of 250 you will get 1.25Amps. The choice is yours.
2n3055 have a min gain of 20. BD* of 40 so as there is about 5mA drive current in the middle, 5 x 40 x 20 = 4A. An Hfe any higher will be wasted.
Just about right for a 2n3055. 2N3773 is a bit of an overkill but similar gain as 2N3055.
Just about right for a 2n3055. 2N3773 is a bit of an overkill but similar gain as 2N3055.
Idle current in TR2 is 29V/3.7K or 7mA.
Peak current in speaker is 29V/8 or 3.6A (3600mA).
If you don't have hFE of 3600/7 or 500, the driver will be unable to pull the load all the way to the rail.
Low end of your specs and Jon's data suggests you could have output stage gain like 40*20 or 800. "It works".
For lowest THD you want the TR3 TR4 drivers to be "very light" loading on TR2. "Very light" might be under 2mA. This demands total output stage gain of nearly 2,000. This could be 40 in each stage, or 80 in the BDs and 20 in the '3773s.
The 3055/3773 devices CAN have hFE as low as 20 around 4A. However most will be higher. Your BDs may have a minimum hFE of 40 but most will be higher.
> the performance of the circuit
Overall this is not what we today call "high performance". Consumer hi-fi moved past this 6-transistor (plus bias) design in the 1970s. However we made a LOT of good sound on amplifiers very much like this one, with similar "just enough" gain and drive. The THD number does not have a lot of zeros in it, but the sound can be good.
"Improved" amplifiers will have several more transistors of higher specs. But since you are concerned about transistor costs, this is not your path for today.
I say just build it. The low-hFE parts will be OK. If BD with hFE >80 are not much more, I would lean that way.
This amplifier design has no Protection. Do not short the output!
Peak current in speaker is 29V/8 or 3.6A (3600mA).
If you don't have hFE of 3600/7 or 500, the driver will be unable to pull the load all the way to the rail.
Low end of your specs and Jon's data suggests you could have output stage gain like 40*20 or 800. "It works".
For lowest THD you want the TR3 TR4 drivers to be "very light" loading on TR2. "Very light" might be under 2mA. This demands total output stage gain of nearly 2,000. This could be 40 in each stage, or 80 in the BDs and 20 in the '3773s.
The 3055/3773 devices CAN have hFE as low as 20 around 4A. However most will be higher. Your BDs may have a minimum hFE of 40 but most will be higher.
> the performance of the circuit
Overall this is not what we today call "high performance". Consumer hi-fi moved past this 6-transistor (plus bias) design in the 1970s. However we made a LOT of good sound on amplifiers very much like this one, with similar "just enough" gain and drive. The THD number does not have a lot of zeros in it, but the sound can be good.
"Improved" amplifiers will have several more transistors of higher specs. But since you are concerned about transistor costs, this is not your path for today.
I say just build it. The low-hFE parts will be OK. If BD with hFE >80 are not much more, I would lean that way.
This amplifier design has no Protection. Do not short the output!
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