Audio Amplifier Design Basics: Problem & Solution

In summary, the discussion is centered around the design of an audio amplifier and the use of biasing resistors in the output stage. It is noted that in the calculations, the base voltage is fixed at 6 volts, but it is not shown how this is achieved. The use of other circuitry instead of resistors is considered more normal for achieving this bias, as the impedance of a biasing network needs to be 1/10 of the input impedance of the emitter follower, which can be difficult to achieve. However, there are advantages to using the direct coupling method, such as fewer components and better low frequency response. The disadvantage is that the operating point may not be as well controlled, but for this particular circuit, it is not
  • #1
erece
72
0
I am studying basics of analog circuit design from microelectronics circuit analysis by donald neamen
And in that i encountered a problem in design of audio amplifier.
The question and solution is given in the attachment.
And i want to know that in the output emitter follower why there are no biasing resistors apart from emitter resistor ?
Hope my question is clear and if not then please do tell me. I want to clarify this
Thank you
 

Attachments

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  • #2
Good question, but it's hard to answer it without seeing the circuit for the complete amp.

You are right in thinking that SOMETHING should be controlling the base voltage of the output transistor.

Maybe the output stage is direct coupled to the stage before, so the previous stage has fixed the base voltage?
 
  • #3
In the calculations in your pdf Vce for the output stage is stated to be 6 volts.

Since Vcc is 12 volts the base voltage is calculated to be (12 - 6) = 6 volts.

thus the base is held at 6 volts (midway betwwwn Vcc and ground)

There must be some circuitry to cause this, it just isn't shown or relevant to the section. Was the chapter on emitter followers (common collector) amps?

Note it is more normal to use other circuitry than resistors to achieve this bias in CC output stages, maybe that is why it was not shown, but will be covered in a later chapter?
 
  • #4
chapter is based on bjt amplifiers...
i agree to both of you , but can you tell me " Why it is more normal to use other circuitry than resistors to achieve this bias in CC output stages ?? "
I think gain stage and output buffer are dc coupled. So operating point of output buffer is fixed by gain stage. Any advantage of this ??
 
  • #5
but can you tell me " Why it is more normal to use other circuitry than resistors to achieve this bias in CC output stages ?? "

The input impedance of an emitter follower is approximately beta times the emitter resistor, which in your case is say 100 x 20 = 2k.

The impedance of any biasing network needs to be 1/10 of this for stability say 200Ω. This is very low and needs to be driven by the previous stage which can be difficult.

So much of the effect of using the emitter follower is lost.
 
  • #6
erece said:
can you tell me " Why it is more normal to use other circuitry than resistors to achieve this bias in CC output stages ?? "
Pass.

I think gain stage and output buffer are dc coupled. So operating point of output buffer is fixed by gain stage. Any advantage of this ??

Fewer components, and better low frequency response since there is no coupling capacitor.

The disadvantage is that the operating point may not be so well controlled when the operating temperature changes, or because of the variation of semiconductor parameters. But for your circuit, it doesn't need to be accurately controlled, because the supply voltage is a lot bigger than the peak-to-peak output voltage you need. A shift of say 1V in the operating point of the output stage isn't going to make much practical difference to anything.
 

1. What is an audio amplifier?

An audio amplifier is an electronic device that increases the amplitude of a signal, making it stronger and louder. It is used to amplify audio signals from sources such as musical instruments, microphones, and audio players.

2. What are the basic components of an audio amplifier?

The basic components of an audio amplifier include a power supply, input signal, amplification circuit, and output signal. The power supply provides the necessary voltage and current for the amplifier to function. The input signal is the original audio signal that needs to be amplified. The amplification circuit is made up of various electronic components such as transistors and resistors, which amplify the input signal. The output signal is the amplified version of the original audio signal.

3. What are the common problems encountered in audio amplifier design?

Some common problems encountered in audio amplifier design include distortion, noise, and thermal issues. Distortion can occur when the amplifier does not accurately reproduce the input signal, resulting in a distorted output signal. Noise can be introduced from external sources or internal components of the amplifier, leading to unwanted sound. Thermal issues can arise if the amplifier is not designed to dissipate heat properly, causing overheating and potential damage to the components.

4. How can I troubleshoot and solve problems in audio amplifier design?

To troubleshoot and solve problems in audio amplifier design, it is important to first identify the source of the issue. This can be done by testing each component individually and checking for any loose connections or faulty parts. If distortion is the problem, adjusting the bias or gain settings may help. Noise can be reduced by using proper shielding and grounding techniques. For thermal issues, ensuring proper heat dissipation through heatsinks and fans can help prevent overheating.

5. What are some important considerations in audio amplifier design?

Some important considerations in audio amplifier design include the desired power output, frequency response, and impedance matching. It is important to choose components that can handle the required power output and have a suitable frequency response for the intended application. Impedance matching between the amplifier and the input and output devices is also crucial for optimal performance. Additionally, considering factors such as cost, size, and efficiency can also impact the design of an audio amplifier.

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