Transistor audio amplifier, HELP! - Page 1
Forum section: electronics / beginners

author: UltraRC
author_rank: Contributor
timestamp: October 05, 2016, 09:46:39 am
content: Up until a few days ago, I thought that when you put a signal into the base of an NPN transistor with the emitter connected to GND and the collector connected to v+ through a resistor, you would get an amplified signal-output on the collector. Nothing I tried worked, and after doing a little more research I found that there was a lot more complexity to the design of an audio amplifier, with concepts such as biasing, the value of Beta, the Quiescent point etc...My situation is that I have an electret microphone circuit which on the output has a signal of roughly 10mV peak to peak, I have been experimenting with very little success for a long time, I think my issue is that I don't make sure what I am trying to do works in theory, I have just thrown a transistor on the bread board and try connecting different things to it hoping that it works (which I confirm is not a good method).Back to the questions:- Does the value of Beta in an NPN transistor change?- What is the quiescent point?- What is biasing, what does it do?- How do I determine the value of the capacitors I use for AC coupling the signal input and output? Up until now, for audio signals I have just been using a 10uF capacitor, is this ok?- How will a load on the output effect the function of the circuit?What I would really appreciate is an explanation of the fundamental and necessary concepts that are needed to use transistors correctly in circuit designs. I know now that a suitable amp configuration for my situation would be a "Common Emitter Amplifier" with a gain of around 100 (with my signal input of 10mV Pk-Pk, I would expect a 1V Pk-Pk signal on the output).ThanksReuben

author: LvW
author_rank: Frequent Contributor
timestamp: October 05, 2016, 09:54:11 am
content: Quote from: UltraRC on October 05, 2016, 09:46:39 amAs the most important aspect: A transistor acts as a voltage-controlled CURRENT source. Hence, you need a resistor between collector and V+ which transfers all collector current changes into the desired signal voltages.

author: setq
author_rank: Frequent Contributor
timestamp: October 05, 2016, 09:56:56 am
content: I think you need to read a book on the matter. I suggest the Art of Electronics or Lessons in Electric Circuits (which is free - just google it).Beta does change. It changes with a few things. Temperature, collector current, frequency. You can't rely on it so don't design specific values or conditions into your circuits. A book will cover this.Quiescent point is when there is no signal so the base is held at whatever bias condition it is set at as is the collector. In a common emitter amplifier, a Q point of 1mA is when there is 1mA flowing through the collector load resistor. The quiescent point is usually described by the amount of collector current flowing.Biasing is complicated to describe. A transistor amplifier has two states: DC and AC. The DC state (biasing) sets the voltages at each of the terminals and therefore with resistors around it, the current (quiescent state) with no signal at all. The AC state of the transistor sets the amplification parameters. Each transistor amp is two circuits running at the same time, the AC superimposed over the top. It's too long to describe here: hit a book.The capacitors for coupling act as filters. On the input, the bias network (voltage divider), the input impedance of the transistor and the coupling capacitor act as a high pass filter. You need to pick the capacitor so that you don't attenuate the low frequency component of your signals. Similar with the output.Load will affect the circuit depending on what the load is and the impedance of previous stages etc. You usually use a common collector (emitter follower) or a push pull topology to provide the necessary impedance conversion.Fundamentally you need to go away and read about the following, in order:1. Loading and impedance. Right back to basic voltage dividers and loading.2. Low pass and high pass RC filters.3. Transistor operating model in detail (particularly the CE amplifier as it's not a current amplifier - it's a voltage controlled current source with a resistive load)4. Push-pull amplifier stages.It's pretty hard to run until you can walk so you need to put some effort into the above to be able to design a transistor amplifier from scratch. I shot myself like you a few times over the years and eventually just sat down with a book and slogged through it for a month.

author: ebclr
author_rank: Super Contributor
timestamp: October 05, 2016, 10:06:15 am
content: - Does the value of Beta in an NPN transistor change?Beta changes according model of the transistor and also temperature.- What is the quiescent point?Is the point where the circuit will be naturally with nothing on input- What is biasing, what does it do?biasing defines the quiescent point, and the region that transistor will work- How do I determine the value of the capacitors I use for AC coupling the signal input and output? Up until now, for audio signals I have just been using a 10uF capacitor, is this ok?The capacitor size depended on the input impedance of next stage, it's a little complex to calculate, 10 uf is a little small will be ok for high frequencies but may attenuate on low frequency using standard transistor.- How will a load on the output effect the function of the circuit?Load on output will affect all aspects, and must be considered alwaysThe is understand that transistor have regions to operate, and you must understand that regions

author: Unknown
author_rank: Unknown
timestamp: October 05, 2016, 10:25:55 am
content: Hi, there are lots of tutorials on the web and some good videos on youtube which go through the operation of common emitter amplifiers and it is an interesting topic to learn. Once you understand how they work and are bored with the maths, I found this tool which is very useful for quickly designing an audio stage:http://en.transistoramp.de/

author: rstofer
author_rank: Super Contributor
timestamp: October 05, 2016, 03:56:50 pm
content: Class A amplifier design is a matter of applying Ohm's Law about 4 times to determine the collector, emitter and bias resistor values. Here is a nice article that gets right down to it using various guidelines:http://www.learnabout-electronics.org/Downloads/amplifiers-module-02.pdfThe rest of the series might make interesting reading but I didn't search for the first module.

author: Connoiseur
author_rank: Regular Contributor
timestamp: October 05, 2016, 04:33:18 pm
content: Here's a quite detailed explanation of everything you need to know about bjt amps.https://youtu.be/Y2ELwLrZrEM