1
\$\begingroup\$

I'm pretty new to electronics, and I need help with designing an amplifier for a specific task. I need to design a bipolar current amplifier using discrete components with a high input impedance that drives a very low impedance load (around 7 ohms). I have tried using a class AB push-pull amplifier, but I've been unable to reduce crossover distortion while maintaining high input impedance and quality output current. Help with this and pointers to educational material regarding similar tasks is appreciated.

Edit: Some comments have requested more information, so here is a schematic. Also, I would like the amplifier to be supplied with 9V DC, and output at least an amp, so a power output of around 7 - 10 watts is desirable. I would like it to operate with a wide frequency range of 60 to 16k hz, and I would like the input impedance to be at least 10k ohms. DC coupled is preferrable, but AC coupled will work. I would like to build the circuit with low level components such as transistors, resistors, caps, etc. Here is a labeled schematic of what I've tried so far:Labeled Schematic

Edit 2: Thank you Jerzy Przezdziecki for your detailed answer, and I will definitely check out your recommended materials, but I'm still having trouble getting this to work. Below is an updated schematic detailing where I am so far. The first section works as intended, but the latter two distort and reduce the amplitude of the signal, and I have no idea how to fix this. If someone could tell me why this unwanted distortion and amplitude reduction is happening and what I need to do in order to make this circuit work as stated in my original edit, that would be greatly appreciated. Updated Schematic, Apologies for Poor Image Quality

\$\endgroup\$
6
  • 1
    \$\begingroup\$ You need to provide more info. Power output? bandwidth? How high is "high input impedance"? DC coupled? \$\endgroup\$
    – danmcb
    Oct 12, 2023 at 7:53
  • 1
    \$\begingroup\$ Welcome. With this type of questions it's always good to provide a schematic and explain the problem and/or what you have tried so far while referring to it. Also, specifically for amplifiers, please provide some necessary design parameters such as the operation frequency range, output power, supply voltage (if there's any available which you are limited to use to). \$\endgroup\$ Oct 12, 2023 at 7:53
  • \$\begingroup\$ @Matthias, You can use either class-A or class-AB (with cross-over issues.) If you don't care about power dissipation then class-A is the approach. There will be no cross-over distortion to worry over. But you will waste a lot of excess power and see more like 25% efficiency in delivering power to the load. The reason for class-AB is that you want to reduce the dissipation and get close to 50% efficiency into the load. There is some crafted design requirements you must meet in creating a class-AB to significantly reduce the cross-over distortion. (But that will cost you some in efficiency.) \$\endgroup\$ Oct 12, 2023 at 9:28
  • \$\begingroup\$ What does 'design' mean? Choose high level IC components, or choose low level transistor components? What power does this 'specific task' require? 10 mW, 1 watt, 100 watts, 10 kW? The best answer differs somewhat given the choice. Is efficiency an issue? \$\endgroup\$
    – Neil_UK
    Oct 12, 2023 at 10:10
  • 2
    \$\begingroup\$ I need help with designing an amplifier for a specific task. Any class-D audio amplifier IC will do this job admirably. Add an inductance in series with the load, since those amps expect that. Designing such a circuit from scratch is a rather involved task, and unnecessary unless you want to spend a week or longer working on it - and not in a simulator, but also on the bench. To see what's involved, go read writings from Pass Labs, Elliott Sound Products, etc. \$\endgroup\$ Oct 12, 2023 at 18:00

2 Answers 2

3
\$\begingroup\$

Without a detailed schematic I can only answer in general terms.

Designing a discrete bipolar current amplifier for a low impedance load can be a challenging task, especially if you're aiming to achieve minimal crossover distortion. Crossover distortion arises when the signal transitions between the NPN and PNP transistors in a class AB amplifier, due to the region where neither transistor is fully conducting. However, there are ways to reduce this distortion.

Here's a basic approach to design an amplifier with minimal crossover distortion:

  1. Biasing for Minimal Crossover Distortion:

    • Use a diode or VBE multiplier biasing circuit to appropriately bias the transistors. This ensures that at least one transistor is always slightly 'on' during the zero-crossing, thus reducing crossover distortion.
  2. Bootstrap Capacitor:

    • Adding a bootstrap capacitor can increase the input impedance by making the base of the driving transistor seem more like an AC open circuit.
  3. Emitter Degeneration:

    • Adding small resistors in series with the emitter of each transistor can linearize the amplifier and reduce distortion.
  4. Negative Feedback:

    • Incorporate negative feedback to linearize the amplifier response further and reduce the output impedance.
  5. Drive Stage:

    • If your signal source cannot deliver enough current to drive the push-pull stage directly, use a driver stage before the push-pull stage.

Here’s a simplified version of a class AB amplifier for your requirements:

  1. Input Stage:

    • This can be a simple differential amplifier using NPN and PNP transistors.
  2. Voltage Gain Stage:

    • This can be achieved using a single transistor. You can use bootstrapping here to achieve a high input impedance.
  3. Output Stage:

    • This is your class AB push-pull stage.

Components:

  • NPN and PNP transistors for differential input and push-pull output.
  • Biasing resistors and diodes/VBE multiplier for the push-pull stage.
  • Capacitors for AC coupling and bootstrapping.
  • Resistors for negative feedback and emitter degeneration.

Once you've set up this configuration, you will need to tune and adjust the values of resistors and capacitors to get the desired performance, keeping an eye on the crossover distortion.

I'm pretty new to electronics

As for educational material:

  1. The Art of Electronics by Horowitz and Hill - This is a comprehensive book that covers a wide range of electronics topics, including amplifier design.

  2. Designing Audio Power Amplifiers by Bob Cordell - This is a deep dive into audio amplifier design and provides a lot of information about class AB amplifiers and the challenges of designing them.

  3. Online Forums: Websites like the EEVBlog Forum, diyaudio.com, and others have extensive discussions and threads on amplifier design, challenges, and solutions.

Finally, consider using simulation software like LTspice, which is free and allows you to test and refine your circuit before physically building it. This will save you a lot of time and components in the long run. Good luck!

\$\endgroup\$
1
  • \$\begingroup\$ If the output voltage is 1V peak then the base of each transistor will be about +1.7V peak. Then each 5k base resistor will have a voltage of 5V - 1.7V= 3.3V and have a current of 3.3V/25k= 0.132mA. Most transistors have a minimum hFE of about 50 then the peak output from each transistor will be 50 x 0.132mA=6.6mA. Then the peak output power into the 7.8 ohms load is only 0.00034W. \$\endgroup\$
    – Audioguru
    Oct 12, 2023 at 19:15
0
\$\begingroup\$

If you reduce the 25k resistors to 180 ohms each then the input impedance is much lower but the output power is much higher. simple amplifier

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.