amp design help - Page 2

wires · 21st October 2010, 06:33 PM

Nope, sorry, right the first time. oops.

wakibaki · 21st October 2010, 08:03 PM

Yes, right.

Your first attempt at the current was correct. The peak value of the current is sqrt(2) times the RMS value and the pk-pk value is 2*sqrt(2) times the RMS value.

W=I^2R. I^2=0.8/8. I=sqrt(0.1)=0.316. *sqrt(2)=~450mA

Unfortunately your problems do not end there.

You have 4.5V available. This equates to a peak voltage of 2.25V and an RMS voltage of ~1.6V. W=V^2(RMS)/R

(1.6^2)/8=0.32W

So even if you can contrive to make your amplifier swing 0-4.5V the output power will not exceed 320mW into 8 ohms.

Power output is where amp design always starts. From this you can calculate the supply voltage required for the topology you have chosen. Some topologies will require more than the basic voltage swing required across the load. If less voltage than this is available then an inductor or transformer will be required in the collector circuit. Inductors are more suited to narrowband applications. Transformers are virtually unknown in BJT audio applications nowadays.

Your best bet is to employ a rail-rail opamp and an emitter follower configuration as I suggested earlier, unless you want to get a higher voltage power supply. Set the opamp output to 1/2 Vcc with a voltage divider at the non-inverting input. AC couple throughout. Set the TIP31 output to 1/2 Vcc with a voltage divider at the base. Make the current through the divider 10 times the quiescent current divided by the TIP31 gain. You can use equal valued resistors and a diode to get 0.6V positive offset. The emitter will settle at the base voltage minus 0.6V. Put a resistor from the TIP31 emitter to ground that gives you slightly more than the quiescent current when the voltage across it is 1/2 Vcc.

w

wires · 23rd October 2010, 02:25 PM

thanks once again waki, you have been a great help, I will try doing that. I have been testing my circuit and it works which is good and it has given me a good base to work with. I will now try what you suggest and also try doing a push pull and see where that takes me.

Thank you.

wires · 07:10 PM

Hello again, i have been working on my AB amp now for some time and i just can't get it to work. The single ended class A amp worked fine. I made a simple preamp using an lm741, and although it was a bit on the quiet side or distorted (nothing a bit of tweaking etc can't fix) it worked.
However, my AB amp doesn't work at all, no sound. I have been working on this for many many hours now and i just can't figure why it doesn't work.
Here's a schematic of it:

The emitter resistors are about 5 or 6 ohms (smallest i have), the transistors are TIP31A and TIP32A, i have used resistors from 0 to 100K ohms for the variable resistor (to play with). The circuit has been tweaked many times and somtime i can get a bit of sound out but its always very very quiet or very distorted and still quite quiet. For example, if i don't connect the collector of the first transistor, and connect the base of PNP to the base of this initial transistor i get noise. But if i take this transistor out of the circuit completely i get no sound.

I really have no clue why this does not work so if anyone could help me i will be increadibly happy.

wires · 15th November 2010, 07:06 PM

sorry, the base of the PNP is connected to the collector of the first transistor, i will edit the schematic now.

wakibaki · 15th November 2010, 07:23 PM

There's no current into the base of the input NPN which is therefore cut off.

w

wires · 15th November 2010, 07:54 PM

Ah. That does make sense. So if i were to have a resistor from V+ to bias it, all will be fine? Having said that, when i have removed the input transistor entirely, i still get close to no sound at all.

wakibaki · 15th November 2010, 08:06 PM

Try 100k. This will give you 40uA into the base, 4mA standing current for an hfe of 100. Now you will have a potential on the 2 bases of the output pair which should keep them turned on (just). Tweak the pot to set the junction of the diodes to 0V. The emitter resistors need to be small (1R) and the caps dimensioned appropriately.

w

wires · 15th November 2010, 08:11 PM

ah wow. Thank you so much!

richie00boy · 15th November 2010, 08:21 PM

A better bias network would be a resistor from each rail to the base. 100k should be OK.

15th November 2010, 07:05 PM	#14
wires diyAudio Member Join Date: Oct 2010	Hello again, i have been working on my AB amp now for some time and i just can't get it to work. The single ended class A amp worked fine. I made a simple preamp using an lm741, and although it was a bit on the quiet side or distorted (nothing a bit of tweaking etc can't fix) it worked. However, my AB amp doesn't work at all, no sound. I have been working on this for many many hours now and i just can't figure why it doesn't work. Here's a schematic of it: The emitter resistors are about 5 or 6 ohms (smallest i have), the transistors are TIP31A and TIP32A, i have used resistors from 0 to 100K ohms for the variable resistor (to play with). The circuit has been tweaked many times and somtime i can get a bit of sound out but its always very very quiet or very distorted and still quite quiet. For example, if i don't connect the collector of the first transistor, and connect the base of PNP to the base of this initial transistor i get noise. But if i take this transistor out of the circuit completely i get no sound. I really have no clue why this does not work so if anyone could help me i will be increadibly happy. Last edited by wires; 15th November 2010 at 07:10 PM.

15th November 2010, 08:21 PM	#20
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	A better bias network would be a resistor from each rail to the base. 100k should be OK. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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21st October 2010, 06:33 PM	#11
wires diyAudio Member Join Date: Oct 2010	Nope, sorry, right the first time. oops.

21st October 2010, 08:03 PM	#12
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	Yes, right. Your first attempt at the current was correct. The peak value of the current is sqrt(2) times the RMS value and the pk-pk value is 2sqrt(2) times the RMS value. W=I^2R. I^2=0.8/8. I=sqrt(0.1)=0.316. sqrt(2)=~450mA Unfortunately your problems do not end there. You have 4.5V available. This equates to a peak voltage of 2.25V and an RMS voltage of ~1.6V. W=V^2(RMS)/R (1.6^2)/8=0.32W So even if you can contrive to make your amplifier swing 0-4.5V the output power will not exceed 320mW into 8 ohms. Power output is where amp design always starts. From this you can calculate the supply voltage required for the topology you have chosen. Some topologies will require more than the basic voltage swing required across the load. If less voltage than this is available then an inductor or transformer will be required in the collector circuit. Inductors are more suited to narrowband applications. Transformers are virtually unknown in BJT audio applications nowadays. Your best bet is to employ a rail-rail opamp and an emitter follower configuration as I suggested earlier, unless you want to get a higher voltage power supply. Set the opamp output to 1/2 Vcc with a voltage divider at the non-inverting input. AC couple throughout. Set the TIP31 output to 1/2 Vcc with a voltage divider at the base. Make the current through the divider 10 times the quiescent current divided by the TIP31 gain. You can use equal valued resistors and a diode to get 0.6V positive offset. The emitter will settle at the base voltage minus 0.6V. Put a resistor from the TIP31 emitter to ground that gives you slightly more than the quiescent current when the voltage across it is 1/2 Vcc. w

23rd October 2010, 02:25 PM	#13
wires diyAudio Member Join Date: Oct 2010	thanks once again waki, you have been a great help, I will try doing that. I have been testing my circuit and it works which is good and it has given me a good base to work with. I will now try what you suggest and also try doing a push pull and see where that takes me. Thank you.

15th November 2010, 07:06 PM	#15
wires diyAudio Member Join Date: Oct 2010	sorry, the base of the PNP is connected to the collector of the first transistor, i will edit the schematic now.

15th November 2010, 07:23 PM	#16
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	There's no current into the base of the input NPN which is therefore cut off. w

15th November 2010, 07:54 PM	#17
wires diyAudio Member Join Date: Oct 2010	Ah. That does make sense. So if i were to have a resistor from V+ to bias it, all will be fine? Having said that, when i have removed the input transistor entirely, i still get close to no sound at all.

15th November 2010, 08:06 PM	#18
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	Try 100k. This will give you 40uA into the base, 4mA standing current for an hfe of 100. Now you will have a potential on the 2 bases of the output pair which should keep them turned on (just). Tweak the pot to set the junction of the diodes to 0V. The emitter resistors need to be small (1R) and the caps dimensioned appropriately. w

15th November 2010, 08:11 PM	#19
wires diyAudio Member Join Date: Oct 2010	ah wow. Thank you so much!