simplest bridge amplifier with transistors

Thread Starter

mike_canada

Joined Feb 21, 2020
239
I'm trying to figure out the simplest way to make a bridged amplifier with minimal part count since someone said you could get double output power for making one. This is my attempt by my output waveforms (blue and green) measured at the collectors don't seem right. Input waveform is yellow.

Is there an easy fix to this?

br.png

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crutschow

Joined Mar 14, 2008
30,768
That circuit will no generate any significant power.

What load are you wanting to deliver the power to?

If you want a bridged amp to power a speaker, then you need to look at audio power amps.
 

DickCappels

Joined Aug 21, 2008
9,297
It is better than you friend told you! You don't just get double the power - you get double the P-P voltage which can quadruple the power delivered to the load.

The basic concept is to make one side go up when the other side goes down. Here is a trivial example for an audio amplifier in which RL represents the speaker:
1663962689925.png


The more detail you can provide about what you would like to try, the more on-target help you can get.
 

LowQCab

Joined Nov 6, 2012
2,501
Do You have a Power-Supply that will reliably deliver
the amount of Power that You expect ?
I see that your Circuit specifies ~14.4-Volts, is this an Automotive application ?
Is this a Mono-Amp, or a Stereo-Amp ?
Do You expect High-Fidelity-Sound, or is some Distortion not really an issue ?
Is this just a noise-maker ?, like a Siren, or a Bull-Horn ?
What is your Budget for this project ?
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Thread Starter

mike_canada

Joined Feb 21, 2020
239
I have 7.2V rechargable batteries and my budget is bad. Distortion is an issue as I want music to be played and amplified as clearly as possible.
 

LowQCab

Joined Nov 6, 2012
2,501
The Fidelity is going to be heavily dependent on the Specifications of the Speaker, and the Speaker-Cabinet.
Speakers normally run with over ~10+% Distortion, and some of them have terrible efficiency.
Please describe the Speaker and the Cabinet with as much detail as possible.

Depending on the Specifications of your Batteries,
You may not want a Bridge-Amp because of
much higher Current demands being put on the Batteries.

What are the Specifications of your Batteries ?, and how much run-time do You expect ?

A very simplistic Transistor-Amplifier is much more likely to produce a lot of Distortion,
as opposed to a proper ~$5.oo Amplifier-Chip.

Are You trying to build this project
out of "Junk-Drawer" parts that You just happen to have laying around ?
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Thread Starter

mike_canada

Joined Feb 21, 2020
239
Please describe the Speaker and the Cabinet with as much detail as possible.
This is the speaker:
https://www.digikey.ca/en/products/detail/visaton-gmbh-co-kg/FR-10-HM-8-OHM/9842400

I don't really have a cabinet.

Depending on the Specifications of your Batteries,
7.2V NiMH 6800mAh batteries (used in RC cars).

And how much run-time do You expect ?
at least 120 minutes per battery.

Are You trying to build this project
out of "Junk-Drawer" parts that You just happen to have laying around ?
Yes. I have mainly 2N3904, 2N3905, 2N2222, TIP41, TIP42, TIP31 and TIP32 transistors as active components.
 

LowQCab

Joined Nov 6, 2012
2,501
You have the same 2 Output-Transistors listed in your Post.
The Input-Stage-Transistors I'm sure can be replaced with something you've got listed,
but I never got into Bipolar-Transistors, so I can't tell You what will work,
maybe someone else here can help You out with exchanging them to other numbers.
For the same reasons, I can't tell you how to easily Drive a second Amp for a Bridge-Amp setup,
but this looks like a very viable Circuit for what You want to do.

https://www.eleccircuit.com/4-transistor-audio-amplifier-circuit/
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5watts-4-transistor-OTL-amplifier-circuit.png
 

Thread Starter

mike_canada

Joined Feb 21, 2020
239
I have explored that circuit and also explored a version without the PNP transistor Q1. I built one similar, but I'm told to get better output power without raising the voltage that I should bridge the amplifier.
 

MrChips

Joined Oct 2, 2009
27,113
Build a cabinet for your loudspeaker.

Without the cabinet the back acoustic wave is going to cancel out the front acoustic wave at low frequencies. There will be no bass response.
 

Thread Starter

mike_canada

Joined Feb 21, 2020
239
ok so other than the cabinet, is there a way to build a simple bridge amplifier? I think with a bridge amplifier I'd get double power for the same battery voltage?
 

DickCappels

Joined Aug 21, 2008
9,297
An enclosure is simple but you need some basic woodworking tools. Back to your topic.
You get double the voltage compared to what you would get from a single-ended amplifier.

With about $3 worth of parts, not counting the power supply, you can get over 30 watts into the speaker. I suggest you get used to the idea of a full bridge using integrated circuits, then later when you know how to design linear transistor audio amplifiers you can use the same technique using your amplifiers of your own design.

Please see the attached datasheet.
 

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Audioguru again

Joined Oct 21, 2019
5,160
The TDA2030A amplifier IC is obsolete and ain't made no more no more.

The little speaker is only 4" in diameter then it will produce not much bass sounds even when in a 125 cubic inch enclosure.
Without an enclosure then the low frequencies from its rear will cancel low frequencies from its front making it a squeaker, not a speaker.

Mike Canada already has a thread about an amplifier powered from 14.4V and driving an 8 ohms speaker.
The 14.4V battery is probably Ni-MH then it produces 16.8V to 18.0V when fully charged so the maximum power before clipping
from the amplifier will be more (maybe 15W).

Here is the amplifier from the other thread but with two amplifiers that are bridged:
 

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DickCappels

Joined Aug 21, 2008
9,297
The TDA2030A amplifier IC is obsolete and ain't made no more no more.

(some text removed for clarity.
I designed some Ultrasonic Technologies (UTC) TA2030's into a product a few years ago. Both the chips and the product are still on the market. My distributor in Bangkok has over 3,000 in stock at U.S 35¢ each. In the context of this thread they are intended as a vehicle for learning. When they finally really do become obsolete no doubt a decent second or third source will appear just as some did for the LM386.
 

Thread Starter

mike_canada

Joined Feb 21, 2020
239
amp.png
This is the closest I can get so far to a working h-bridge amplifier. The middle 8-ohm resistor is meant to be a speaker but I don't think speakers can be better simulated. Now I gotta make some tweaks so the voltage swings from 0-14 instead of from about 1/2 to 13.
 

DickCappels

Joined Aug 21, 2008
9,297
"Gotta" is slang and not widely recognized around the world.

This one is a definite improvement!
Off hand, I can spot two areas for possible improvement.

If you want your output stage to be biased for little cross-over distortion, put low value emitter resistors (0.5Ω to 2.0Ω) in series with the output transistor emitters. They will be useful in stabilizing the bias current through the transistor. Without the emitter resistors the transistors can easily overheat -as die temperature increases, base-emitter voltage decreases and emitter current goes up. The resistors help minimize that effect.

How stable are the DC output voltages on each side of the bridge? That could be a problem if the DC voltage across the speaker drifts. You really want it as close to zero volts DC and zero V/°C as reasonably possible.

Also, have you checked the bandwidth to see whether it is sufficient for your needs?
 
The TDA2030A amplifier IC is obsolete and ain't made no more no more.
I think the TDA2040 is still current and really inexpensive. Although I do understand the desire to build from scratch using discrete components this integrated amplifier is so worth it if you value your time. The datasheet (attached) does all the work for you, even with an example of a bridge amplifier. As previously mentioned, getting the power supply and the speaker(s) right is where the time should be spent!
 

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