issue generating 20mA current.

Thread Starter

asbjc

Joined Oct 30, 2020
32
Hi !

I'm working to simulate a problem (driving 4ohm 10Watt speaker from AB-Amplifier).
I'm using LM741 to first amplify the voltage(0.2v) a bit, then power amplifier to increase the power for 10watt speaker.
I'm done with required output results.

But, in simulating it on LTspice, i've issue generating 20mA signal from voltage source.

Can anyone help me with a way to theoretically make calculations/ adjustments to generate the 20mA signal for input. (for simulation)

Actually, a hit & trial gives 20mA at 20V input (as shown in picture)... While 200uA at 0.2V
How can i get 20mA from 0.2V (without disturbing inverting amplifier circuit) ?


The circuit is as follows:
 

Attachments

Audioguru again

Joined Oct 21, 2019
2,406
Your circuit will not work.

The lousy old 741 opamp is 52 years old with awful performance. Use a newer opamp designed for audio or use a new audio power amplifier IC. Your 50V power supply will destroy 741 and most other opamps, why is it so high?
Why are your resistor values so low? An audio amplifier has a 50k input resistance, yours is only 1k.
Why do you need 20mA? 0.2V into 50k is a current that is only 4uA.
Your second opamp does nothing and can be replaced by a piece of wire.
Your power amplifier has no voltage gain.
Your power transistors have their bases connected together producing class-B with lots of crossover distortion, not class-AB.

10W into 4 ohms is 6.33V RMS which is 17.9V peak-to-peak. Use a power supply that is plus and minus only 12V.

10W into 4 ohms has a peak current of 9V/4 ohms= 2.25A. The minimum current gain of a TIP31 and TIP32 is about 12 at 2.25A so the output current from the opamp must be 2.25A/12= 188mA which is way more than the old 741 opamp and most other opamps can produce. The simulator might produce much more current than real opamps and real transistors can produce, look at minimum values in the datasheets.
 

Thread Starter

asbjc

Joined Oct 30, 2020
32
thanks.


i made some changes, took 0.1V input and amplified it using NPN first. (to 6.1V). Output is exactly 6.1V. (peak)

but issue is, as the output is connected to the AB power amplifier.
it goes to mV.

Why is so ? And how to prevent this happening ?

Also, if input of 6V is applied directly to AB-amplifier,
it works perfect now.

But not from the input from CE amplifier.
 

Attachments

Thread Starter

asbjc

Joined Oct 30, 2020
32
hi,
Post your asc file.
Why didn't you use the 20mA source method I suggested.?
E

i'm first trying to figure out the basic one first, then i'll try moving to the 20mA method you described earlier.
Thanks alot btw. It looks quite helpful
 

Attachments

Thread Starter

asbjc

Joined Oct 30, 2020
32
hi,
Where on the asc circuit are you measuring the 6.2v:???

Please use Idents to indicate the measuring points
E
6.1 V (amplified from 0.1V) is after the C3 (when not connected any further).

It downs to mVs after connection with AB-power amplifier
 

Thread Starter

asbjc

Joined Oct 30, 2020
32

Attachments

Thread Starter

asbjc

Joined Oct 30, 2020
32
hi a,
The 741 is at its maximum current output, ie: so the waveform is clipped.
You must increase the input impedance of the power amplifier stage.
E
Hi,
I tried another opamp, (LT1886) before AB-amplifier.
The results look good to me.
(I needed 10W across 4ohm speaker). It's approximately the same.

Is it correct now ?
 

Attachments

Thread Starter

asbjc

Joined Oct 30, 2020
32
hi a,
This shows the response of your circuit.
E
Oh, i don't really know, whats the issue now... I just measured Power manually, i thought it was fine.

I'm OK with the little less power average, As the 10W required is the maximum value.
But the ups and downs are bothering me.
:confused:
 
Top