# My amplifier doesn't work

Hi! I have tried to create an amplifier from transistor, capacitors and resistors. It has as input the D5 pin from an Arduino Nano. I have programmed the Arduino to output sound with the tone() function. The problem is that the sound doesn't get amplified. I would like it to be around 9V. It is also a bit different from the initial one. Could you please help me solve this problem?
I will attach the schematics of the circuit.

Hi,
OPs circuit.

With no signal, measure the DC voltage between gnd and the collector of the transistor, it should be about 4.5V.
Where did you get the circuit from?
R1 and R2 look very low in value for bias resistors.

Also place a 10uF capacitor between the emitter and gnd, this will increase the AC gain of the amplifier while keeping the thermal runaway protection of the circuit.

Tom...

What are you driving with it?

TomGeorge:
Hi,
OPs circuit.

With no signal, measure the DC voltage between gnd and the collector of the transistor, it should be about 4.5V.
Where did you get the circuit from?
R1 and R2 look very low in value for bias resistors.

Also place a 10uF capacitor between the emitter and gnd, this will increase the AC gain of the amplifier while keeping the thermal runaway protection of the circuit.

Tom...

Hi, Tom!
Thank you for your reply! I have got the documentation from here: How to Build a Voltage Amplifier Circuit with a Transistor, but I tried to make it suite my project, so using the formulas from there I tried to calculate the values for my case. Unfortunately I have 1.8V between the GND and collector.

aarg:
What are you driving with it?

I am trying to drive a speaker. (Sorry if I misunderstood the question)

Looks like you want something like this:

eBay!

Paul__B:
Looks like you want something like this:

eBay!

Thank you, but is there any way to make mine work?

What sort of a speaker? How big is the speaker? What power is needed to drive it? How/where is it connected?

What you have is a voltage amplifier but it will deliver almost no power into a load like a typical low impedance speaker. Did you see this comment on that page you linked to "Therefore, with this circuit, as is, it won't be enough to power on a device due to the high output impedance."?

Steve

slipstick:
What sort of a speaker? How big is the speaker? What power is needed to drive it? How/where is it connected?

What you have is a voltage amplifier but it will deliver almost no power into a load like a typical low impedance speaker. Did you see this comment on that page you linked to "Therefore, with this circuit, as is, it won't be enough to power on a device due to the high output impedance."?

Steve

I don't know almost anything about the speaker. I took it from a YY-610 bike alarm and I couldn't find its specifications on the internet. The circuit was powered with a 9V battery and this is why I thought I should supply the speaker with 9V.

andstef:
I don't know almost anything about the speaker. I took it from a YY-610 bike alarm and I couldn't find its specifications on the internet. The circuit was powered with a 9V battery and this is why I thought I should supply the speaker with 9V.

Assuming it's an 8 Ohm speaker (or even a 40 Ohms, which is about as high as is typically found for a coil type speaker) then the speaker impedance is much lower than that amplifier. If one views the transistor as an ideal switch and ignores the output capacitor, you have 8 Ohms in series with 470 Ohms when the transistor is on and 8 Ohms in series with 4.7k Ohms when the transistor is off. Viewing this as a voltage divider, it's clear that virtually all of the 9V is dropped across the resistor and not the speaker.

This circuit isn't appropriate to drive a low impedance load.

MrMark:
Assuming it's an 8 Ohm speaker (or even a 40 Ohms, which is about as high as is typically found for a coil type speaker) then the speaker impedance is much lower than that amplifier. If one views the transistor as an ideal switch and ignores the output capacitor, you have 8 Ohms in series with 470 Ohms when the transistor is on and 8 Ohms in series with 4.7k Ohms when the transistor is off. Viewing this as a voltage divider, it's clear that virtually all of the 9V is dropped across the resistor and not the speaker.

This circuit isn't appropriate to drive a low impedance load.

Thank you for the explanation. I have measured the resistance of the speaker and it is 310 Ohms.

You should be able to reduce the value of Rc and Re and get some amount of signal out, but it's going to be terribly inefficient. If you're driving a speaker with a fixed amplitude square wave, an H-bridge circuit might be more appropriate.

andstef:
Hi! I have tried to create an amplifier from transistor, capacitors and resistors. It has as input the D5 pin from an Arduino Nano.

Your circuit has an approximate voltage gain of 10x, but you are feeding it a signal that's already 5V peak
to peak, so it will be completely saturated (you may fry the transistor even).

Also that circuit has a 4k7 output impedance, 3 to 4 orders of magnitude out for driving a speaker.

To feed a speaker from a logic signal you need a current buffer, not a voltage amplifier.

Very well, let's get practical. You actually have a 2N2222 then? And want to drive it from an Arduino using "tone"?

OK, I am gong to presume an 8 Ohm speaker. Let's allocate it 100 mA as a safe amount. 5 V over 100 mA is 50 Ohms. Put a 47 Ohm resistor in series with the speaker, connect that series pair to 5 V (forget the 9 V, not necessary ) and to the 2N2222 collector, emitter to ground, base to the Arduino pin via a 470 Ohm resistor.

Do I have to draw that for you?

If you can't recognise the title, check here.

Paul__B:
Very well, let's get practical. You actually have a 2N2222 then? And want to drive it from an Arduino using "tone"?

OK, I am gong to presume an 8 Ohm speaker. Let's allocate it 100 mA as a safe amount. 5 V over 100 mA is 50 Ohms. Put a 47 Ohm resistor in series with the speaker, connect that series pair to 5 V (forget the 9 V, not necessary ) and to the 2N2222 collector, emitter to ground, base to the Arduino pin via a 470 Ohm resistor.

Do I have to draw that for you?

If you can't recognise the title, check here.

Thanks! It would be nice to have a diagram.