Stages of audio amplification from mV to 20V+

Question

I have been trying to learn how to amplify audio signals from very small signals (20mV) to large signals (20V+) while using only transistors. I have figured out how to build a class A common emitter amplifier that can take a few mV to 1-2V. However, I don't know how to increase the signal higher.

I have read that you can't just take a 1-2V signal and increase it to 20V because it has to be in stages. My question is what are the stages to increase the 1v signal to 20v? Will a class A amp work by increasing VCC or are other classes better (class ab, class B…)? How many stages do I need? I am very confused about this and would like someone to point me in a direction.

Answer 1

An audio amplifier generally has 3 stages, the input stage to buffer the weak signal to have weak voltage with low output impedance that can drive the next stage, usually a voltage amplification stage which provides the gain in amplitude and low enough impedance to drive the next stage, usually the output buffer stage for driving the amplified voltage with required current and low enough impedance to be able to drive whatever load is next.

So you need a gain of 1000 to go from 20mV to 20V, which is huge gain of 60dB. No single stage can do that as it would require enormous bandwidth from the amplifier. But since a gain of 32 is easier, you need two stages with gain of about 32 to have total gain of 1024. That's only 30dB per stage, easily achievable with op-amps.

Depending on what exact audio source you are amplifying (i.e. source impedance) and what load you are driving (e.g. speaker), you may need to split the gains differently and design the stages differently if input stage needs to be somehow special regarding noise or other source characterisrics, and select a suitable op-amp for that purpose.

And op-amps can't drive speakers so again if you add gain with a second op-amp, you need a high current output stage. Maybe an amplifier chip able to drive speakers directly, instead of building one from discrete transistors.

Answer 2

You're asking for a gain of 1000 overall. You will need to break that up into several stages.

A brief proposal would be:

• Low-noise, AC coupled low-level preamp, gain of about 50 ~ 100, max swing about 2V
• Intermediate (line-level) stage, gain of about 10.
• final driver (power) stage with low gain (about 2.)

Breaking this down further, stages 2 + 3 should be designed together as a line-level amplifier block with negative feedback. 2V in would give your maximum output (20V). So a reasonable design would be a gain of 10 for this. Lots of references for discrete designs, including the Blomley one I've posted below.

Meanwhile, Stage 1 would be a microphone preamplifier or similar. This would have an impedance-matched stage appropriate for your source (electret, dynamic, phono...) with low-noise gain and be able to drive a line-level output. Internally, this might also be more than one stage: a low-noise diff amp stage and a line driver stage. I would recommend an op-amp for this as your first go.

To round out your design you probably want some additional features like gain control.

At any rate, line-input power amplifier designs are easy to find. Study these and understand them. You could try your hand at a highly-regarded discrete design that I simulated.

Again, I'm guessing that a full discrete is more than you're prepared to deal with on your first go. Try an op-amp based one first.

Answer 3

It all depends on exactly what you want to accomplish. You can certainly design a Class A circuit that will amplify a signal of a volt or two up to 20 volts in a single stage, but it would tend to have a high output impedance so would not drive something like a speaker.

So you have to define what you want. Ask yourself these questions:

• What is my signal source? What is it's impedance and what is it's voltage range?
• What is my load? What is it's impedance and what is the voltage range I expect across it for the given source range?

When defining your voltages you will need to specify them in a consistent manner. Are they peak voltages? Peak to peak? RMS? This will be important when it comes to your supply rail voltages and transistor ratings. 20 V\$_{pk-pk}\$ is 56.6 V\$_{RMS}\$, quite a difference so you need to know which you're dealing with.

Also, if you're going to be working with amplifiers a lot you will want to learn how to use decibels to express voltage and power ratios.

If you want to take the output of the amplifier you have and drive a speaker, there are plenty of designs out there. One common type is a differential pair for the input stage, a Class A amplifier for the voltage amplifier stage (commonly called the VAS) and a Class A-B complementary symmetry output stage. These will easily take a 1 V\$_{RMS}\$ input and drive a speaker to 20 V\$_{RMS}\$. A good place to start looking at circuits is diyAudio.

If you just want to increase the voltage and aren't worried about power (if you're driving a high impedance instead of a low impedance) a couple of stages of Class A amplifiers should work as long as you have supply voltages that allow them to reach the output voltage swing you want, and keep in mind the transistor voltage ratings.

Answer 4

The days of home stereo sets have probably been over for a decade or two, but I suggest that you take inspiration from the blockwise structuring of those signal chains. An important interface was the "line level" between a preamp + tone controls block, and the power amp (PA) block. This "line level" was something like 1 Volt AC. In practice, ancient stereos were working with a line level more like 300 mV, while more recent models were up to maybe 5V AC line level. But, let's say that the PA can work with up to 1V AC input and has a couple dozen kiloOhms of input impedance.

In your preamp and tone controls stages, you need to have enough voltage gain to reach the "line level".

In the PA, you need enough voltage gain and current gain to drive an electrodynamic speaker (4-8 Ohms) to the Watts you envisage :-) If you know the peak Watts desired and the Ohms of your speakers, you can calculate peak voltage (AC RMS). Multiply by approx. two, to get the DC rail voltage, required for a class AB power stage to achieve your desired wattage - and, you will need two such rails (+ and - , relative to your common ground) or a setup where your speaker is bridge-tied between two output "totems".

To get some practical schematics of a PA block, I suggest that you use google images.

Most of the PA schematics that you'll find are going to be class AB = two output linear transistors, one pulling up, the other one pulling down, with some non-zero quiescent current "in the middle point". In those schematics of a PA block, you can typically identify three inner stages:

1. an input stage - typically a proper diff input based on a long-tailed pair

2. a voltage gain stage - such as a single common-emitter BJT

3. the output buffer stage - typically a cascade of common collector BJT's (or darlington pairs). You can speak about a driver sub-stage and the very output power substage (the totem). This whole buffer stage has a voltage gain lower than 1, but provides a significant current gain - to be able to drive a speaker that has just a few Ohms of nominal impedance.

The PA block typically uses an inner negative feedback - to set a fixed and manageable gain, to minimize distortion, to achieve reproducible overall parameters in manufacturing etc. The negative feedback is why you need a differential input in the PA block.

Recommended reading to initiate you gently into some typical circuit building blocks: Hans Camenzind - Designing Analog Chips . The book is available online in PDF for free. Feel free to skip the parts on silicon doping and etching, and go straight to the circuit topologies. IIRC there is something on the long-tailed pair, the current mirror, a constant current source, a very simple op-amp (and much more). It's a follow-up to the "transistor amplifiers 101" (common emitter, common collector, common base).

Answer 5

it doesn't have to be in stages, but transistors suffer from being real devices do other things bedides amplifying (eg givving differen perfromance depending on the supply voltage or the temperature.

The emitter resistor in a common emitter amplifier introduces negative feedback and this trades gain for linearity, this reducing distortion.

To go from 2v to 20V is basically the same as from 20mv to 2V, only you need a higher supply voltage.