# TIP31 Biasing Problems?

#### crazyengineer

Joined Dec 29, 2010
156
Okay, so once again, I created another orcad simulation so I can try biasing a TIP31 transistor. I wanted to bias so that the ac waveform entering the bas will be centered around 8 volts However, when I made a plot of the ac voltage entering the 8 ohm resistor, the shape of the 2.83 AC signal entering the base was recreated, but it was reduced somewhere around the millivolt range.The red line indicates the DC voltage at the collector, and the green line indicates the AC voltage at the 8 ohm resistor. Here's a larger picture of the ac voltage Did I screw up with my biasing or is this one of the problems of a class A amplifier: the fact that the efficiency is much less than other amplifiers?

Joined Dec 26, 2010
2,148
If you look closely, you will see that the peaks of the loudspeaker waveform do not coincide with those at the collector. There is an obvious reason for this.

What is the reactance of a 10μF capacitor at 100Hz? Is it low compared to the speaker's 8Ω impedance? If not, what might be done about this?

Edit: There are other issues here, but let's deal with one thing at a time.

#### crazyengineer

Joined Dec 29, 2010
156
If you look closely, you will see that the peaks of the loudspeaker waveform do not coincide with those at the collector. There is an obvious reason for this.

What is the reactance of a 10μF capacitor at 100Hz? Is it low compared to the speaker's 8Ω impedance? If not, what might be done about this?

Edit: There are other issues here, but let's deal with one thing at a time.
Let's see

The reactance of the capacitor (assuming xc=1/(2*pi*f*C) is 159.55 when a 100 hertz signal is applied. Since the capacitor is connected in series with the 8 ohm resistor, I need a smaller capacitor in order to get more ac power into the resistor. Am I correct?

Last edited:

Joined Dec 26, 2010
2,148
Let's see

The reactance of the capacitor (assuming xc=1/(2*pi*f*C) is 159.55 under a 100 hertz wave. Since the capacitor is connected in series with the 8 ohm resistor, I need a smaller capacitor in order to get more ac power into the resistor. Am I correct?
Not quite: Xc is inversely proportional to C.

Joined Dec 26, 2010
2,148
I said this, among other things, in reply to an earlier thread:

...It is normal to avoid passing DC through the speaker. One very common way to do this is by putting a capacitor in series with it. Many hundreds to thousands of microfarads are required, depending on how low the speaker impedance is, and how much bass response is required. If you have only a small loudspeaker that only responds down to a few hundred Hz a few hundred μF may do...
But 10μF is too small.

#### w2aew

Joined Jan 3, 2012
219
Part of the problem is that you are using a Common Emitter amplifier. A CE amplifier's output impedance is essentially equal to the collector resistance. Thus, when you load it with a low impedance like 8 ohms, you completely attenuate it's output.

To drive a low impedance like a speaker, you have to use an amplifier with a low output impedance. There are a number of ways to do this:

1) Use a audio transformer in the collector as your load, with a turns ratio that creates a reasonable collector impedance on the primary side and a low impedance for the secondary for the speaker.

2) Use the CE amp as you have shown, but follow it with a second stage that can drive a low impedance load - something like a common collector (emitter follower) circuit.

3) etc...

#### crazyengineer

Joined Dec 29, 2010
156
But 10μF is too small.
Part of the problem is that you are using a Common Emitter amplifier. A CE amplifier's output impedance is essentially equal to the collector resistance. Thus, when you load it with a low impedance like 8 ohms, you completely attenuate it's output.

To drive a low impedance like a speaker, you have to use an amplifier with a low output impedance. There are a number of ways to do this:

1) Use a audio transformer in the collector as your load, with a turns ratio that creates a reasonable collector impedance on the primary side and a low impedance for the secondary for the speaker.

2) Use the CE amp as you have shown, but follow it with a second stage that can drive a low impedance load - something like a common collector (emitter follower) circuit.

3) etc...
Okay! I see my mistakes. Although I'll take w2aew suggestion about using a common collector configuration in future designs, I still want to play around with this circuit some more.

Here's the updated schematic with a 470u capacitor instead of the 10u VC and 8 ohm resistor voltage plots 8 ohm voltage plot closer up 