What's wrong with my transistor amp ? - Page 1
Forum section: electronics / beginners

author: wexeteme
author_rank: Contributor
timestamp: February 23, 2015, 08:55:26 pm
content: Hello, recently I have started having fun with electronics and so I kinda tried to design amp from a transistor, and here's the schematicThis is updated version of the post, now I am facing signal noise at high frequencies, so how do I fix that ?Please tell me what I am doing wrong, and please don't be angry on me I am just a beginner!Thanks!

author: KJDS
author_rank: Super Contributor
timestamp: February 23, 2015, 09:03:25 pm
content: It is a truth universally acknowledged that any transistor without a 220R resistor in series into the base will oscillate at a frequency just above what you are capable of measuring.

author: wexeteme
author_rank: Contributor
timestamp: February 23, 2015, 09:17:55 pm
content: Thanks for replaying @KJDSas you said, I added 220oms resistor at the base in series, but nope, the same results ...

author: Andy Watson
author_rank: Super Contributor
timestamp: February 23, 2015, 09:21:41 pm
content: Examine the bias point of the transistor. I make the base to be about 2V, if the transistor has a reasonable amount of hfe it will be turned on - full stop - no room for the collector voltage to swing.

author: wraper
author_rank: Unknown
timestamp: February 23, 2015, 09:40:27 pm
content: What is the load connected? Don't you try to drive some low impedance headphones with it?

author: wexeteme
author_rank: Contributor
timestamp: February 23, 2015, 09:45:24 pm
content: The load is ~6oms speaker.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 23, 2015, 10:07:31 pm
content: Quote from: wexeteme on February 23, 2015, 09:45:24 pmAh, I see! You didn't mention the loadYour "amplifier" will have an output impedance that is essentially the same as the resistor in the collector, i.e. 1k, and you are driving this into 6 ohms!Edit: Make that 1k plus the impedance of the 1uF at whatever frequency you're operating at.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: February 23, 2015, 10:44:06 pm
content: I have a transistor amplifier on my youtube channel made from 4 transistors, an op amp, and a couple resistors. It is an npn/pnp push pull design, with a second set of identical transistors connected as diodes thermally attached to the output transistors. This creates a very good bias voltage for the output transistors, and the opamp creates the gain. This design is capeable of driving DC loads too, as there are no caps (aside from voltage rail caps). The youtube channel username is the same as my username here and the video is a homemade oscilliscope (see where that DC drive comes in handy?)

author: wexeteme
author_rank: Contributor
timestamp: February 23, 2015, 11:24:17 pm
content: Okej. so I know I am behind, but can you please give me short notes of how input and output impedance affect the apm, shoul input be bigger then the output or the other way around, and how can I increase or decrese my Input and Output impedance in the circuit above, thanks!

author: w2aew
author_rank: Super Contributor
timestamp: February 24, 2015, 01:31:20 am
content: Quote from: wexeteme on February 23, 2015, 09:45:24 pmPut that 6ohm load in your simulator, and you'll see what happens!Try sticking and emitter follower between your collector and the output coupling cap (as a starting point).

author: Tim F
author_rank: Regular Contributor
timestamp: February 24, 2015, 06:13:26 am
content: There's nothing actually wrong with your common-emitter amplifier, it's just not suited to driving a low impedance load like a 6ohm speaker. The output impedance is too high (1Kohm) to efficiently drive a 6ohm load. Almost all the power is dissipated in the amplifier instead of in the load - imagine connecting a perfect amplifier through a 1Kohm resistor to your 6ohm load. It forms a resistor divider, almost all of the voltage gets dropped across the 1Kohm resistor and you get a very low voltage across your 6ohm load as you have observed. You'd probably only want to use this amplifier to drive a load that has an impedance of 10Kohm or more.What you want is to drive your speaker with an amplifier that has an output impedance significantly lower than 6ohms, so you need to add a current amplifier stage - an amplifier which has a voltage gain of 1 but provides a high impedance input which is easy for your common-emitter amplifier stage to drive and a low impedance output to effectively drive your speaker. As above, an emitter-follower amplifier (aka common-collector amplifier) will do this.Also keep in mind that the output coupling cap will form a highpass filter with the speaker with a corner frequency of f=1/(2*pi*R*C) which gives you 26KHz for your 1uF cap and 6ohm speaker so very little signal in the audible range will pass. You want f to be atleast 300Hz for voice and as low as possible (preferrably around 5-10) for bass/hifi audio. Using a 100uF cap will give you flat response down to 265Hz with your 6ohm speaker.Better yet, place a Class AB buffer at the output which avoids the need for a large value cap altogether:The input cap doesn't need to be 100uF. 1uF would do just fine there because its connected to a high(ish) impedance and since f=1/(2*pi*R*C) if you make R larger, C can be smaller.

author: wexeteme
author_rank: Contributor
timestamp: February 24, 2015, 07:02:08 pm
content: Alright, I added 200k resistor at the output and attached the speaker parallel to this resistor and I kinda get some increase in the sound although it has a lot of distortion at higher volume.Anyway, thanks for replying I guess I learned something!

author: lapm
author_rank: Frequent Contributor
timestamp: February 25, 2015, 09:47:01 am
content: Quote from: wexeteme on February 24, 2015, 07:02:08 pmDistortion or clipping?

author: wexeteme
author_rank: Contributor
timestamp: February 26, 2015, 02:27:55 pm
content: well I dont really know, but it doesn't matter since I kinda know whats the problem, the thing is the signal swing, at high frequencies the signal top swing is about 1 volt and the bottom one is like -3volts, meaning the top signal goes in to saturation which aint good, so I need to redesign the circuit and make that signal swing equal I guess.

author: wraper
author_rank: Unknown
timestamp: February 26, 2015, 08:26:18 pm
content: Quote from: wexeteme on February 26, 2015, 02:27:55 pmNo, few people already told about this, but you didn't get it at alll.. The issue is you cannot drive low impedance load with high impedance signal source. And this amp circuit is exactly high impedance signal source. You can attach 1k resistor in series between any normal amplifier and your speaker and will see exactly the same result. Adding 200k resistor in parallel with the speaker is just LOL, IMO you should recreate some amp schematics already avalable before trying to design anyting. Maybe this is harsh but you have zero understanding about this circuit so far.

author: wraper
author_rank: Unknown
timestamp: February 26, 2015, 08:49:58 pm
content: In addition to the previous post. This circuit have few other issues of course but they are not as important the main issue with impedance which make this circuit completely unusable.

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2015, 08:57:33 am
content: Yes, as everyone has stated the amplifier has too higher impedance to drive a 6 Ohm speaker.This means that the output of the amplifier effectively has a 1k resistor connected in series with it. Put a 1k resistor in series with a 6R load and you have a potential divider. The voltage divider will reduce the output voltage by a factor of 6/1006 = 0.005964 so if the output from the amplifier is 2V before the speaker is connected, it will fall to 11.93mV as soon as the load is connected.

author: wexeteme
author_rank: Contributor
timestamp: February 28, 2015, 01:10:25 am
content: @wraperwhen I said in pararell I meat in series my bad on that I am sorry, and yea I got no understanding on this, in fact that's the main reason why I wrote this post :"C--------------------------------------------anyway, I updated the post, so my output signal gets noise (high freqs) at the top signal swing, what's going on, how can I fix this ?

author: sarahMCML
author_rank: Regular Contributor
timestamp: February 28, 2015, 04:47:51 am
content: Quote from: Andy Watson on February 23, 2015, 09:21:41 pmTry changing the value of R3 to 39K in your edited circuit and re-measure the bias points again (without an input signal). Then apply a signal and see what it looks like on your scope!If you compare the before and after bias values you should get some useful ideas about why the first version doesn't work well. It still needs more work to drive a loudspeaker, as others have suggested, but this should give you a start.Regards,Sarah.

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2015, 03:16:23 pm
content: It's possible to make a speaker amplifier with one transistor but it'll be crap.Attached is an example. It will work but there's a high DC current through the speaker (up to 200mA, so make sure it's rated to handle the power dissipation, 0.5W minimum when you add the signal), a low input impedance and there will be some distortion.It also uses some high value capacitors but they only need to be rated to a low voltage so they won't be that big.It's not a very good circuit. You're better off using more transistors or a speaker amplifier IC.