AF amplifier idea

John_1981 wrote:Thanks for the link to the transistor biasing info, good explanation on bootstrapping and easy to understand. Do i still need to bootstrap if i replace R2 with a current source?

Hi John,
The constant current source has a low impedance, so the answer is no.
Bootstrapping is intended to solve the resistive bias solution.

Hafta go, Maj-Lis sis houting for me  

Best regards from Harry - SM0VPO

Thanks for the link to the transistor biasing info, good explanation on bootstrapping and easy to understand. Do i still need to bootstrap if i replace R2 with a current source?

John_1981 wrote:I seem to recall finding a good explanation on bootstrapping somewhere on your homepages. Could you point me in the right direction? my understanding is that the voltage from the output stage is 'stacked' on top of the supply voltage to give increased drive. Why is this not also done for the negative going half of the cycle?

Having done some more experimenting, I have found that doubling the power using bridge mode is hardly worthwhile as I can barely hear the difference in output, the only real advantage is to do away with the output capacitor but at the expense of increased circuit complexity.

http://85.226.187.247/begin/bias-10.htm

That is the link you want. I builds up the amplifier, and there is a section about bootstrapping.

Why is this not also done for the negative going half of the cycle?
This is because R2, R3 and C2 are in the positive side, and it is R2 that is the problem. As the output voltage waveform increases towards peaks, so the voltage across R2 falls. The current reduces and there is less drive to TR2 base. That is why the peaks are the ones that sag. With low voltages in the waveform, so the current through R2 is maximum, but TR1 takes it AND all the current TR3 needs.
 
In amplifiers with dual power supplies (+12 and -12) then the same problem can occasionally exist, depending on the circuit configuration. 

I have found that doubling the power using bridge mode is hardly worthwhile as I can barely hear the difference in output
By using the push-pull complementary pair you get double the voltage into the load, AND double the current. This is a gain of 4X (6dB). But you can also lower the speaker impedance from 8 Ohms to 4 Ohms and make that 8X the power. You will notice the difference between 4 Watts and 32 Watts, if you wind up the volume. So will your neighbours 

Very best regards from Harry - SM0VPO

John_1981 wrote:... doubling the power using bridge mode is hardly worthwhile as I can barely hear the difference in output ...

Doubling the power = 3 dB increase in volume. It is not very much.

Using an IC containing a pair of AF PAs (originally designed for sterero) makes the job much easier.

VBR from Ivan

I seem to recall finding a good explanation on bootstrapping somewhere on your homepages. Could you point me in the right direction? my understanding is that the voltage from the output stage is 'stacked' on top of the supply voltage to give increased drive. Why is this not also done for the negative going half of the cycle?

Having done some more experimenting, I have found that doubling the power using bridge mode is hardly worthwhile as I can barely hear the difference in output, the only real advantage is to do away with the output capacitor but at the expense of increased circuit complexity.

Thanks for the interesting info,
I would be interested in seeing the circuit. I did something very similar a few years back using two pairs of TIP31/32 and putting the speaker between the emitters of the two output stages. The object was to eliminate the huge capacitor required for the speaker, and to quadruple the AF output power available from a 12V car battery.

Before this I used a split-supply (12-0-12V DC) to drive a single output stage, with the output biased to 0V. That way the speaker could be connected directly to ground without any capacitor. Only problem was that the output power available in a car was low because it is limited to a single 12V source, which is why I tried the 2-pa amplifier.

I didn't use operational amplifiers. I used bipolar devices throughout. I will see if I can dig out the circuit from the old lab book. It must have been about 1999 or so, but the circuit of one side is similar to the 4-Watt AF amp on my homepages.

Best regards from Harry

Hi John,
this principle is used quite often. Can you give us your schematic, please?

BR from Ivan

Having tried out a few of the AF amplifiers on HHH, I thought I would try something different. I made a couple of push-pull complementary pairs using 2SC945 and 2SA733's, each driven by one half of a TL072 dual op amp, then wired the loudspeaker in bridge, so in effect I have a class AB 'H' bridge. The input to each op amp is driven by a phase splitter, I have used both the long tailed pair and a single transistor with equal collector/emitter resistors but not sure yet which i prefer. The amplifier produces a reasonable amount of power using a low supply voltage (9V) and without needing power transistors.