# Amplifying Signals generated from phone

Discussion in 'The Projects Forum' started by Mozee, Jul 26, 2016.

1. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
I am trying to build an amplification circuit to amplify signals generated from a function generator application by a smartphone so that the signal is powerfull enough say 10volts and 2A in such a way that the wave form doesn't change (a small change won't kill me though).I am still new to op-amps so bear with me please.

I read something about push pull transistor configuration to draw current to feed the powered signal. I will appreciate it if someone could suggest a proper circuit for my needs keeping in mind that the signal source is a smartphone which needs protection against voltages(currents) flowing to its audio jack.
Also, I would like to ask, If I used an audio amplifier circuit and injected the signal, would it work like that? If so, How can i protect my phone?
How would Darlington power transistors help me in this circuit (because I have them laying around).

2. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,033
1,991
If you want to build a circuit from scratch, there are a ton of amplifier schematics on the innergoogle. Or you can buy an audio amplifier module on ebay.
1. What is the shape (sinewave, squarewave, etc.) and frequency range of the signal you want to amplify?
2. What is the load the amplified signal is to drive?
3. Yes, an audio amplifier should work like that. The most common problem is not connecting the phone's signal wires correctly. There are pre-made adapter cables you can buy for connecting a phone to an audio system.

ak

3. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
Hey Ak, thanks for the reply
The android app called Function Generator can produces sine, square and saw teeth wave shapes therefore those are to be amplified. The wiring is already done easy and simple.
Frequency range would basically be the audiable range but the higher the frequency the better the circuit serving my needs.

I am trying to build a function generator that can produce at least sine and square waves at decent controllable frequencies, the higher the better, However
, what I have laying around is a LM324N as an op amp to be used.

I've seen many circuit diagrams but I seem to have this problem of finding the common,+ve and -ve connections. I would appreciated if you could give me a circuit so I can build it and disscuss any issues the might face me to learn where are my mistakes. Also I am open for another Idea for a function generator

4. ### wayneh Expert

Sep 9, 2010
14,784
5,274
Unless you really want to build something, you'd be far better off just getting an audio amplifier module. See Parts Express if you prefer to avoid EBay. You can buy something ready-made for less than you can make your own, and it will likely be more advanced.

5. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,033
1,991
Since you are using the term function generator, know this - the earphone output of a phone is AC coupled, so all outputs signals will have the 0 Vdc point somewhere in the middle of the waveform (vertically). For example, a square wave won't go from 0 V to 4 V; it will go from -2 V to +2 V if the duty cycle is 50%, - 1 V to + 3 V if the duty cycle is 75%, etc. And even if the phone did make a DC stable output, an audio power amplifier will be AC coupled at its input.

ak

Mozee likes this.
6. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
Thank you friend, problem is thatI live in a country where no country would ship to it less than 60USD (by DHL or UPS etc..) not even china, so It will be cheaper to build it

7. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
Hey AK, thank you for the note. I am totally aware of that, in fact, I would like to have an AC current wave form so I can feed it into an LC circuit for example and use my homemade oscilloscope to measure Inductance of unknown coils and measure unknown capacitance as well as observe how are different components behave and act to different wave forms and frequencies. so I would like to have a fairly powerful signal amplified from a less powerful signal generated from my smartphone using signal generator applications.

8. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,033
1,991
Having a clear idea of what you want makes things easier. 10 V at 2 A implies a 5 ohm load impedance. So you need an audio power amplifier capable of making at least 20 W into a 4 ohm load (these numbers are rounded up or down to standard audio component values). Or, an opamp output booster circuit that runs on +/-15 V and has 5 A output transistors. The bottom schematic on this page:
http://forums.parallax.com/discussion/149035/simple-pwm-to-analog-circuit-0-10vdc
has one general approach.

ak

9. ### wayneh Expert

Sep 9, 2010
14,784
5,274
OK, I see. Do you have parts available? As @AnalogKid noted in #2, there are many audio amplifier schematics available using many different integrated circuits. The simplest is probably the old LM386 chip. You can find that chip in many cheap computer speaker sets, for instance. There are also many more advanced options for use in automotive systems.

10. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
Thank you @AnalogKid and @wayneh
Regarding the suggested circuit, unfortunately I couldn't find this op amp here, however, while googling I found a very similar and interesting project using the LM324 quad op-amp and some power transistors HERE . I tried building the circuit on a breadboard but I noticed that the voltage measured across the ground and the left/right pins(rings) of the TRRS audio plug which is supposed to be plugged into the smartphone reads 8 volts which will definitely fry my valuable smartphone in inserted. I can't understand whats wrong with the circuit or what did I do wrong or could it be the LM324 itself is damaged somehow?

I'd appreciate it if you could have a look at the schematics

11. ### Alec_t AAC Fanatic!

Sep 17, 2013
8,656
1,975
A faulty/incomplete negative feedback loop around IC1B or IC1C could leave you with a large DC voltage on the phone connector. AC coupling to the phone connector via a cap would prevent that.

12. ### wayneh Expert

Sep 9, 2010
14,784
5,274
Yup, if it was my smartphone, I'd feel a lot better using a strictly capacitive coupling. A typical 0.1µF ceramic cap is probably adequate.

13. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
@Alec_t @wayneh

Sorry for the stupid questions I'm still trying to learn here so please bear with me guys, I really appreciate your help.
1- What is a faulty or incomplete negative feedback loop? what do you mean by this term? is it like an asymmetry in the + and - voltages regulated by the regulators? I mean like one is producing +8V and probably the other is producing slightly lower or higher voltage than -8V and vice-versa so it's not a precise -+8 Volts ?
2- How to AC couple the phone connector? Is it by placing a capacitor in series with the non-ground(Common) line e.g. the left/right or both channels lines? and more important, why do we do that? I know that capacitors passes AC signals not the DC ones.

By the way, I fed the circuit with two different power sources (ATX PSU and a normal multi voltage transformer both connected in series each set to regulate 8Volts so I had the -8V +8V and Common.
If I started understanding these things It will help me A LOT with OP-Amps !.. My only problem facing me in Electronics right now at this stage is dealing with Op-Amps in circuit which needs (-Ve ,+Ve and common) lines

14. ### Alec_t AAC Fanatic!

Sep 17, 2013
8,656
1,975
If, for example, the connection to pin 6 of IC1b were missing, there would be no negative feedback to the opamp.
Yes. E.g a 0.1uF cap (as Wayneh suggested) connected between R6 and Phone_audio_left_output_1.

15. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
Thank you @Alec_t I will rebuild the circuit on my breadboard and see what happens and tell you guys.

16. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
@Alec_t @wayneh

I am really confused. I rebuilt the circuit on a breadboard and I am still getting the same 8V across the common line and right and left channels of the TRRS phone plug. I guess there is something wrong with the schematics and the oscilloscope built at the IC1A schematics is not functioning as I can't see any signal when connecting my phone. More importantly the pin 7 of the LM324 opamp was not used as an output of the IC1B, instead, pin 4 which is the V+ was used, so when I connected the 100K pot assembly of the first function generator to the pin7 instead of pin4 (See the IC1B schematics) the voltage across Phone_Audio_Left_Output-1 and Common dropped to some millivolts which should be fine if plugged into the headphone jack and I measured the voltage across common and the signal_gen_left_output-2 It was some millivolts(also fine). However, as soon as I plug the headphone jack and use the function generator app the voltage across ignal_gen_left_output-2 and common rises to about 6 volts(even without using the func. gen. app) but using multimeter to measure frequency produced by it indicated correct frequency but when I plug a 8ohm 1watt speaker I can't hear anything as if there is no frequency?!!
Also the right function generator schematics is still producing 8Volts across the Common(GND) and Phone_Audio_right_Output-2 which is dangerous to the phone.
@ IC1D, the pin13 is indicated as +Ve Input where it is actually a -Ve input to the op-amp (D) as indicated in the below provided figure of the LM324 pinouts

I really don't have much knowledge about the meaning of +ve or -ve inputs and how should they be connected to the other components so here I am asking what could be wrong with the schematics he provided other than that? an explanation would be great so I can learn, if possible of course!

LM324 OP-AMP PINOUT

17. ### Alec_t AAC Fanatic!

Sep 17, 2013
8,656
1,975
So am I. It sounds as though you may have a problem with the power supplies. You don't show how they are interconnected. You can connect them in series (stacked) only if their outputs are independent to start with, i.e their negative terminals don't share a common ground connection.
An LM324 can't power an 8 Ohm speaker directly; you are virtually short-circuiting the poor IC's output .
The post #10 circuit was clearly not designed with an LM324 in mind. The pin-out of the opamps is different. Can you post a corrected schematic showing all the pin-numbers that you are now using?
It would probably be best if you were to start again with a breadboard layout of just one opamp stage, e.g. IC1b. Get that working before adding other stages one at a time.

Last edited: Jul 30, 2016
18. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
@Alec_t

The power source I am using now is two batteries in series, one reads about 7.88V and the other is about 8.2 volts, i series they measure 16volts
so using this combination I was able to get common, -ve and +ve voltages for the op-amp. I think It won't have a big effect on the circuit, right?

The design is not mine as you know and the guy made this schematics stated that the used chip was a LM324 Op-amp :\
I really feel sorry for my lovely LM324 being shorted out I wish she can forgive me for that as I had no idea i was upsetting her, but how am I able to test the output frq. in practical if I can't use for example a speaker to hear the tone or should I use a higher impedance one?(although the multimeter reads the correct frequency when connected).

I will re-breadbord the circuit step by step and see where are things getting weird. stay tuned

19. ### Alec_t AAC Fanatic!

Sep 17, 2013
8,656
1,975
Try applying the signal to the 'line in' input of an audio amplifier or PC sound card.

20. ### wayneh Expert

Sep 9, 2010
14,784
5,274
Or use an old earpiece like they used for crystal radios. Could be tough to find one these days.