Design Amplifier

[Berna Cagley]

Electronicamplifiers can be classified in many ways. They can offer high input impedances, low output impedances, they can have a variety of different bias and operational modes. High power, low noise, class A, class B, class C and so forth. Each type is chosen to suit a different application.

An amplifier can be made in many ways. They can use bipolar transistors, field effect transistors and even thermionic valves / vacuum tubes. The amplifiers can be included within some form of circuit block or integrated circuit. They can even be in the form of operational amplifiers, op amps.

An amplifier can be considered as a block that has two input terminals and two output terminals. As the ground connection is normally common to input and output there are often only three terminals: input, output and the common.

Input resistance - Rin: The input resistance is the resistance that is seen by a signal source when it is applied to the input of the amplifier. The input resistance will become a load to the source. The case where the load is purely resistive is a special case, and more normally it will be an impedance. However for the purposes of this explanation it will be considered to be resistive.

Output resistance - Rout The output resistance is the resistance that can be considered to be within the amplifier as shown below. It will form a potential divider network with any load that is applied to the amplifier. Again the output will have indicative ad capacitive elements which mean it will be an impedance, but for most low frequency applications and for this explanation, it can be considered to be resistive.

The output resistance can be determined by measuring the output voltage under a no load condition, and then a loaded condition, i.e. with the applied load. Knowing the open circuit voltage and the load resistance and the voltage dropped across the internal resistance under load it is possible to determine the source output resistance.

Often within an amplifier the waveform may be inverted, and this is represented by the fact that the gain is negative. In other words if the amplifier had an absolute value of gain of 5, but it inverted the signal, for a 1 volt input, the output would be - 5 volts, and when entered into the equation, this would give a gain of -5.

It is also possible to have current gain within a circuit. This is particularly useful when needing to drive a low impedance load. It is necessary for the current level to be increased, often keeping the voltage the same. Circuits like bipolar transistor emitter followers, FET source followers, op amp buffers with 100% feedback, and for this using tubes / valves the circuits used for this are typically cathode followers.

When using a circuit to provide current gain, it is often necessary to ensure that the circuit has sufficient drive capability. Whist the circuit may be able to provide the level of current gain for low levels of current, in some cases they may not be able to provide high levels of current that may be required in some instances. Using a very obvious example, a small op amp buffer would not be able to drive a large loudspeaker on its own.

Reference to amplifier classes including Class A, Class B, Class C, Class AB and others will often be seen when investigating the form of amplifier. When designing an amplifier, the class is often one item that will appear early in the design cycle.

Amplifiers are one of the most widely used circuits - they are used for audio, DC, radio frequency and very many other applications. They are one of the most common analogue circuits. There is a huge variety of circuits, whether used with op amps, bipolar transistors, FETs of even the old vacuum tubes / thermionic valves.

The tool will help get you close to finding the correct op amp to use in your design but it is always good to verify with simulations and calculation. I recommend taking a look at the below pieces of content for guidance in your design.

I sort of glanced at those Tim, they are kind of bouncing around looking at stability mainly I think, If you set out to deliver a butterworth closed loop response, the transimpedance design flow is pretty simple, take the circled equation here and solve for the GBP given your desired feedback R and F-3dB and total Cs on the inverting summing junction,

Also Tim, I have gotten pretty tired of that 40dB closure rate caveat, not as hazardous as folks think, I covered that at the end of this article using a transimpedance design - essentially, one way to think about it is the closed loop 2nd order transimpedance response Q is set by the feedback pole (while the Fo is set by the geometric mean of the noise gain zero and the op amp gain bandwidth product). So just sliding that feedback pole around on the NG curve (by adjusting Cf) is changing the response Q, only. So, for instance, if you set the pole equal to Fo (at the intersection of the NG and Aol curve) you will get a Q=1 in the closed loop transimpedance design (ADI has done this for years, apparently not really realizing that is what they are doing). And, if you then take 1/2 the Cf, you will get Q=2, peaking of course, but not unstable as most folks seem to think even though that is giving a 40dB loop gain closure rate.

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:

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.

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.

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.

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!

I can suggest that u have to try a simple circuitry using bigger transistor with higher supply voltage & current if u can. Keep on experimenting safely & don't hesitate to ask coz there are a lot of generous people that willing to share their knowledge & ideas. I was just an ordinary electronics fella! Do u guys believe that I successfully developed a 3 transistor class AB amplifier into 30 ohm to zero impedance load. Using same number of transistor of the original schematic. My circuit runs on 18 - 40vdc or even higher without heatsink on 3 ohm load continuous.

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For the SS Series the main design focus was to pack as much full-range stereo output capability as possible into as compact of a chassis as possible. We utilized discrete circuit class D technology for ultra low distortion levels that rival class AB designs. The resulting amp is capable of driving multiple driver mid-range and tweeter arrays while consuming less current, taking up less space and delivering more power.

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