Hello All
Refer to picture below.
Could you let me know what is purpose of resistor in the red circle ?
I guess this may be feedback to Vbe it is correct ?
Thank you.
Best Regard
Refer to picture below.
Could you let me know what is purpose of resistor in the red circle ?
I guess this may be feedback to Vbe it is correct ?
Thank you.
Best Regard
That's correct. The 470Ohm resistors set the output stage gain to 1+470/56=9.39 by feedback.
Douglas mentioned this structure in Audio Power Amplifier Design (6th Edition) P241. Rod Elliott wrote about this output stage with gain design in a publicly available article.
Douglas mentioned this structure in Audio Power Amplifier Design (6th Edition) P241. Rod Elliott wrote about this output stage with gain design in a publicly available article.
Think of the BD139 and BD140 as like opamps, with emitter as inverting input, base as non-inverting input! The output transistors act as current buffers in the loop.
The problem with this circuit is the low impedance the feedback networts have to be, which means they typically have to dissipate substantial power, several watts.
The problem with this circuit is the low impedance the feedback networts have to be, which means they typically have to dissipate substantial power, several watts.
I just noticed the opamp needs a bias resistor between its + input and ground, eg 47k.
Due to I use supply that have current maximum 500mA.
How to adjust the resistor or where in circuit that have to change ?
How to adjust the resistor or where in circuit that have to change ?
The short answer is your power supply has inadequate available current.
Assuming the supply current limits when the peak current reaches 0.5A, the peak voltage developed into the 8 ohm load will be 0.5A * 8 ohm = 4V peak. This implies sine wave clipping will begin at 1W RMS. Roughly, 35V/8 ohm = 4.375A peak and the corresponding sine wave power would be 75.56W RMS. This corresponds to 35V peak into the 8 ohm load with no drop in supply voltage.
These are theoretical numbers that presume lossless circuit performance. In practice, the design in your OP wouldn't perform very well, but it is interesting conceptually.
Assuming the supply current limits when the peak current reaches 0.5A, the peak voltage developed into the 8 ohm load will be 0.5A * 8 ohm = 4V peak. This implies sine wave clipping will begin at 1W RMS. Roughly, 35V/8 ohm = 4.375A peak and the corresponding sine wave power would be 75.56W RMS. This corresponds to 35V peak into the 8 ohm load with no drop in supply voltage.
These are theoretical numbers that presume lossless circuit performance. In practice, the design in your OP wouldn't perform very well, but it is interesting conceptually.
The voltage gain of the circuit can be modified, but that won't help the dilemma with power supply vs. output power.
I'm sorry, but I don't understand what you are suggesting/asking.
I'll the same thing a little differently. The power supply constrains the maximum power that can be delivered to the load: if the supply has ample current, the peak power is controlled by the supply voltage; if the supply voltage drops because it has reached a current limit, its current limits the peak power.
The voltage gain of the amp controls only where you have to position the preamp volume control. The AC gain is (1+R2/R3) or about 48 in the posted circuit.
I'll the same thing a little differently. The power supply constrains the maximum power that can be delivered to the load: if the supply has ample current, the peak power is controlled by the supply voltage; if the supply voltage drops because it has reached a current limit, its current limits the peak power.
The voltage gain of the amp controls only where you have to position the preamp volume control. The AC gain is (1+R2/R3) or about 48 in the posted circuit.
Just saw your post. I suppose you could reduce amplifier gain so that you could not drive the amp into clipping. But the your maximum sound pressure level (SPL) would still be disappointing. Doing that would be analogous to putting a speed governor on your car, or having the gas pedal reach the floorboard before engine reached redline.
You need a more powerful supply.
You need a more powerful supply.
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I do to reduce the gain because My speaker is small (8 ohm 12 watt) and I already have PCB circuit in the workbench.
OK I will try.
OK I will try.
Fun circuits to play with.
CFP has stability issues as is.
With added feedback has its drawbacks.
These circuits can work and work well.
And enjoyed playing around with them.
But you just got to be careful and takes a little time
to stabilize these circuits.
problem with this one.
Very familiar with, since it appears often in amplifier
searches on the net.
It does not work. As mentioned needs additional resistor on opamp input.
Then when it does work.
The circuit will be unstable.
Additional compensation capacitors
are needed.
The resistors set the gain of the feedback of the CFP section circled in red
is a little to high.
And the total gain of the amplifier is also a little high.
Unless that is desired.
CFP has a lot of cross conduction at high frequency.
Better matched output devices can help.
2N3055/2955 being seen in many classic early amplifiers.
Are extremely poor choice and not well matched for CFP output stage
and especially with additional feedback
CFP has stability issues as is.
With added feedback has its drawbacks.
These circuits can work and work well.
And enjoyed playing around with them.
But you just got to be careful and takes a little time
to stabilize these circuits.
problem with this one.
Very familiar with, since it appears often in amplifier
searches on the net.
It does not work. As mentioned needs additional resistor on opamp input.
Then when it does work.
The circuit will be unstable.
Additional compensation capacitors
are needed.
The resistors set the gain of the feedback of the CFP section circled in red
is a little to high.
And the total gain of the amplifier is also a little high.
Unless that is desired.
CFP has a lot of cross conduction at high frequency.
Better matched output devices can help.
2N3055/2955 being seen in many classic early amplifiers.
Are extremely poor choice and not well matched for CFP output stage
and especially with additional feedback