Questions about AB class amps / output buffers

[atferrari]

I am about to build an output stage for a sine generator.

I run across several circuits of AB class amplifiers but in spite of lot of reading I still have many basic doubts.

My questions:

1) A buffer output is expected to have gain by itself? In other words what defines the gain in a pushpull configuration?

2) Several function generators seem to set their output impedance just by using a series resistor of 50 / 75 / 600 ohms at the output. Is that all is needed?

3) Instead of building an AB stage, the LH0033 (or similar buffer), would be a better (if expensive) solution?

4) When reading about AB amps it seems that those specifically for audio and those used as an output stage of signal generators share different necessities. Besides the obvious low impedance loads in audio, is anything that make them so different?

5) Walter Jung in his IC Op-amp Cookbook says that the attached circuit has an output impedance of 10 Ohms. What determines that value? How could I change it, if possible?

6) Read somewhere in a forum that diodes as shown above are old use; that a Vbe multiplier should be used instead. Can anyone elaborate?

Gracias for any help to understand this a little more.

 

[audioguru]

1) A buffer output is expected to have gain by itself? In other words what defines the gain in a pushpull configuration?

No. A buffer has unity gain. The driver p[rovides voltage gain.

2) Several function generators seem to set their output impedance just by using a series resistor of 50 / 75 / 600 ohms at the output. Is that all is needed?

If the load is a low impedance then a class-AB output must provide enough current.

3) Instead of building an AB stage, the LH0033 (or similar buffer), would be a better (if expensive) solution?

Use a good audio amplifier IC.

4) When reading about AB amps it seems that those specifically for audio and those used as an output stage of signal generators share different necessities. Besides the obvious low impedance loads in audio, is anything that make them so different?

Audio amplifiers have low distortion and low noise.

5) Walter Jung in his IC Op-amp Cookbook says that the attached circuit has an output impedance of 10 Ohms. What determines that value? How could I change it, if possible?

I think the circuit has an extremely low output impedance but it has a fairly low maximum output current of only 100mA.
The 22 ohms emitter resistors reduce the maximum output level into a load so maybe 10 ohms is its minimum load.

6) Read somewhere in a forum that diodes as shown above are old use; that a Vbe multiplier should be used instead. Can anyone elaborate?

Gracias for any help to understand this a little more.[/QUOTE]

 

[KeepItSimpleStupid]

I have to disagree slightly with your answer to #1. The output stage provides a voltage gain of 1, but it has a measureable current gain. The beta of the transistors.

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2. Think about it. The OP amp circuit ususllay has a very low output Z, so 0+(the outout Z required) works. A resistor in the output tends to isolate capacitive loads which can make the amp unstable. Feedback is then taken after the resistor. lot of times it's an easy way to implement short circuit protection. In audio, the gismo needs to be able to drive a 600 ohm load at the 1V RMS level minimum. 75 and 50 are needed for high frequencies. Feedback lowers the output Z, but "I missed that lecture"

3) I used the LH0002 for a nice buffer with one of the "capacitive isolator" circuits from the app note. I also built a simpler version of the discrete circuit. Remember the diodes have to be in intimate contact with the output devices.

4) They usually have a Zobel network at the output which isolates the inductance. It is usually a carbon resistor with a wire wrapped around it. They have an RF filter at the input and the output. The frequency response is taylored to audio and operation to DC is removed.

5. The Vbe multiplier does a better job because it's adjusaible. The need for it at VI of (10V)*(0.1A) or 1 W is very questionable.

 

[audioguru]

A resistor in the output tends to isolate capacitive loads which can make the amp unstable. Feedback is then taken after the resistor.

No.
Feedback is taken before the series output resistor. Then the resistor can isolate the feedback from the output capacitance.

 

[atferrari]

Which ones do you suggest?

Use a good audio amplifier IC.

Audio amplifiers have low distortion and low noise.

Aren't audio amps optimized for low loads like 4 , 8 or 16 Ohms? If I load one with a 600 Ohm resistor what could be the result?

BTW which ones would you suggest? Two or three options would help when checking what I could obtain around here?

Gracias for your time.

 

[bountyhunter]

Aren't audio amps optimized for low loads like 4 , 8 or 16 Ohms? If I load one with a 600 Ohm resistor what could be the result?

Driving a higher impedance load is no worries.

 

[audioguru]

A modern amplifier works perfectly with its rated load or no load.
An old vacuum tube amplifier blows up if its load impedance is too high.

 

[unclejed613]

1)a buffer amp has no voltage gain, but has current gain

2)what you set as the output impedance with a series resistor depends on what you intend doing with it. a 600 ohm output impedance was the standard for audio many years ago because a lot of audio test equipment was designed for telephone systems. 50 ohm is pretty much standard for most test equipment these days. in the test equipment industry, the same 50 ohm attenuators, splitters, etc... are used for RF and audio equipment, so it makes sense to have test equipment all meet the same standard. 75 ohm outputs are standard for broadcast and CATV equipment. some computer monitors used to have "50 ohm" bnc video connectors but the input impedance was 75 ohms.

for audio it really doesn't make much difference, until you start driving low impedances with the signal generator. most inputs on audio equipment are terminated with 10k, and line level outputs are 1k impedance, with the concept here being "voltage matching" where 1.5Vrms is the "standard" line level.

3) a diy buffer stage isn't difficult to make, or expensive.

4)most solid state amps (i.e. those that drive speakers) have output impedances measured in milliohms. most of the low impedance is due to negative feedback and the open loop gain of the amplifier. if you missed the class, i wrote a paper with experiments here: **broken link removed**

5)it's actually lower than that because of feedback, but because of the 22 ohm resistors in the output (paralleled they would be 11 ohms), the amplifier begins to run out of steam if the load impedance goes below 10 ohms.

6)use either, but thermally couple either the diodes or the Vbe multiplier transistor to the output transistors' heat sink