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bridge

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Electronman

New Member
Hello,
I am wondering if it is possible to make a bridged amplifier with op-amps?
I want to have a bridged op-amp to get more output power.

Thanks.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hello,
I am wondering if it is possible to make a bridged amplifier with op-amps?
I want to have a bridged op-amp to get more output power.

Thanks.
You don't get more power from a bridged amp anyway, you merely get the same power but feeding a different impedance.

You can certainly bridge opamps, it's probably simpler than poweramps - and will give you the same double the output VOLTAGE - the current doesn't alter though (obviously).
 

audioguru

Well-Known Member
Most Helpful Member
Yes you can bridge two opamps. But the ouput power from one opamp is very low and the output power from two bridged opamps is also very low.

An opamp is spec'd to have a minimum load of 2k ohms because its output current is limited to a max of only 30mA.

An LM386 is a little power amplifier, not an opamp. It can drive an 8 ohm speaker because its output current is limited to 375mA. Its output power is 0.25W into 8 ohms at clipping with a 6V supply. It is not bridged.

A TDA2822M is a little power amplifier that is bridged. Its output power is 0.8W into 8 ohms at clipping with a 6V supply.
 

Hero999

Banned
You don't get more power from a bridged amp anyway, you merely get the same power but feeding a different impedance.
That's not true.

You get double the power.

The output voltage swing is doubled but the output current remains the same so the minimum impedance you can drive is also doubled.
 

Electronman

New Member
You don't get more power from a bridged amp anyway, you merely get the same power but feeding a different impedance.

You can certainly bridge opamps, it's probably simpler than poweramps - and will give you the same double the output VOLTAGE - the current doesn't alter though (obviously).
I did not get your mean Nigel
 

Electronman

New Member
Ok here's my design of an amplifier made by Op-amps.
The output is an ultrasonic sensor ideally with the bandwidth of more than 10kHz.
Please tell me your opinion and or any optimization recommendation.
Do I need any coupling cap at the output too?
Can I get 28V ideally at the load?

P.s. I do not know how much of output it is able to give me though.
 

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Electronman

New Member
Yes you can bridge two opamps. But the ouput power from one opamp is very low and the output power from two bridged opamps is also very low.

An opamp is spec'd to have a minimum load of 2k ohms because its output current is limited to a max of only 30mA.

An LM386 is a little power amplifier, not an opamp. It can drive an 8 ohm speaker because its output current is limited to 375mA. Its output power is 0.25W into 8 ohms at clipping with a 6V supply. It is not bridged.

A TDA2822M is a little power amplifier that is bridged. Its output power is 0.8W into 8 ohms at clipping with a 6V supply.
audioguru,
I need it for an ultrasonic transducer as I told at above post.
First I designed it by the bellow circuit but the output was not so good for the ultrasonic sensor. I hope to get more power with Opamps (like my above design) tan this circcuit.
 

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audioguru

Well-Known Member
Most Helpful Member
The outputs of most opamps do not go as high as the supply voltage nor go as low as 0V. They have a saturation voltage loss.
Most opamps cannot directly drive the fairly high capacitance of a piezo transducer without a series resistor that will also reduce the output level.
Your bridged opamp circuit will have an output of about 24V p-p into a 10k piezo transducer with a 14V supply.
 

Electronman

New Member
So what's the solution?
lease tell me if I am able to use those audio amplifiers as my circuit driver (the output freq goes more than 50kHz at some situstion).

What about using TDA2822??

If you were me what did you do?
 

Electronman

New Member
That's not true.

You get double the power.

The output voltage swing is doubled but the output current remains the same so the minimum impedance you can drive is also doubled.
Please tell me how bridge does work?
I think after bridging the impedance of the load remains constant. I think what will happen is that the voltage across the load will be 2 times when bridging and finally causes the current into load to be doubled too, right?
 

audioguru

Well-Known Member
Most Helpful Member
I would use an MC34072 dual opamp to drive your piezo transducer in a bridge. It has an ouput swing of about 27V p-p driving 10k ohms with a 14V supply and can directly drive a high load capacitance. It performs perfectly up to 100kHz.
 

Electronman

New Member
I would use an MC34072 dual opamp to drive your piezo transducer in a bridge. It has an ouput swing of about 27V p-p driving 10k ohms with a 14V supply and can directly drive a high load capacitance. It performs perfectly up to 100kHz.
So it does not have the bridge problems you mentioned at the other post about other opamps working on bridge mode??
 

audioguru

Well-Known Member
Most Helpful Member
A TL071 opamp and many others oscillate when they drive only 100pF of capacitance. The MC34072 does not oscillate when it drives 10nF.

Most opamps have a saturation voltage loss of 3V. The loss for an MC34072 is only about 1V.
 

Sceadwian

Banned
Bridging is pretty simple to understand. Instead of grounding one lead and driving the other with a power driver, you drive both ends with a power driver, you just feed one driver an inverted signal.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Please tell me how bridge does work?
I think after bridging the impedance of the load remains constant. I think what will happen is that the voltage across the load will be 2 times when bridging and finally causes the current into load to be doubled too, right?
Only if the amplifier can supply enough power - as I said before a bridged amplifier can only provide exactly the same power as the two individual ones, only the load impedance changes.

I'll give you a common example:

You have two 100W 4 ohm amplifiers, and two 4 ohm speakers.

You connect one speaker to each amplifier, and get 100W in each speaker, total 200W.

You now bridge the same two amplifiers, and connect the same two speakers in series across the output (giving 8 ohms), you get the exact same 100W in each speaker, and the exact same total 200W.

Where bridging gives an advantage is if you don't have the correct impedance speakers - say you only have one speaker, 8 ohm 200W. You connect the speaker to one of the amplifiers, but you now only get 50W from it - by using the two amplifiers bridged, you get the full 200W as before.

If you connect a 4 ohm speaker to the bridged amp, it will try and provide 400W to the speaker (200W from each amp) and both amplifiers will be destroyed.
 

Hero999

Banned
I see what you're getting at now Nigel, you are correct: bridging doesn't give you any more power per amplifier, it just eneables you to put more power into higher impedance loads.
 

Sceadwian

Banned
Bridging in that aspect is pointless, it's smarter to run two amps to the two loads separately. But it's a TOTALLY different picture when you're talking about one speaker being driven from a single ended amp vs bridged, the power does double, it however does require an amplifier made for bridging or for the two separate amplifiers to be isolated.
 

unclejed613

Well-Known Member
Most Helpful Member
the amps don't need to be isolated, you just don't use the grounds. when you drive an amp in bridge mode, there's a "virtual" ground in the center of the load. basically, each amp in the bridge "sees" half of the load impedance. if one of the amps in the bridge were to lose signal and just sit there at 0V output, that amp output would become a "virtual" ground. in fact if you were to monitor the base drive of the "ground" amp, you would find that the amp is "working" to keep that output voltage AT zero. there's no real purpose in running an amp in this manner, but it does reveal some of the inner workings of amplifier feedback theory.

if you want to drive a 4 ohm load with a bridged amp, make sure the amps used are stable to 2 ohms. most of the time if a 100W/ch stereo amp drives 8 ohm speakers to 100W, you will only get 150-175W in bridge mode driving an 8 ohm speaker. this is because of voltage sag in the power supply and the higher voltage drop across the output devices due to the increased current.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Bridging in that aspect is pointless, it's smarter to run two amps to the two loads separately. But it's a TOTALLY different picture when you're talking about one speaker being driven from a single ended amp vs bridged, the power does double, it however does require an amplifier made for bridging or for the two separate amplifiers to be isolated.
No, the power quadruples, but only because it was running at half power from the single ended amp, as my example above - due to using the wrong impedance speaker.

My PA amp will provide 400W RMS per channel in 4 ohms, or 800W RMS mono in 8 ohms for bridged mode. I generally feed 2 x 8 ohm speakers from it, so run at 'only' 200W per channel.
 

Electronman

New Member
Only if the amplifier can supply enough power - as I said before a bridged amplifier can only provide exactly the same power as the two individual ones, only the load impedance changes.

I'll give you a common example:

You have two 100W 4 ohm amplifiers, and two 4 ohm speakers.

You connect one speaker to each amplifier, and get 100W in each speaker, total 200W.

You now bridge the same two amplifiers, and connect the same two speakers in series across the output (giving 8 ohms), you get the exact same 100W in each speaker, and the exact same total 200W.

Where bridging gives an advantage is if you don't have the correct impedance speakers - say you only have one speaker, 8 ohm 200W. You connect the speaker to one of the amplifiers, but you now only get 50W from it - by using the two amplifiers bridged, you get the full 200W as before.

If you connect a 4 ohm speaker to the bridged amp, it will try and provide 400W to the speaker (200W from each amp) and both amplifiers will be destroyed.

That's because those speakers you mentioned became IN SERIES and so the impedance is doubled so the bridge had nothing to do. But suppose you bridge those amplifiers and connect just one of 4ohms speakers across them, now because you have feed the 4ohms speaker with a DUBLE voltage (I.e. the bridged amplifier) the current that is going to go into the speaker is doubled too and so the speaker works more powerful. My assumption would be wrong only if you tell when bridging the impedance of the load will be doubled automatically!.

I do not know why both speakers would be destroyed but it tells that my assumption is right.
 
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