Gbp

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soundman

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Hello guys,

Any idea of the effect of bridging two OP_AMPs on Gain Bandwidth Product?
I have two amplifiers and GBP is 100 kHz for each. What will happen to the GBP if I bridge these 2 OP-AMPS?

Thanks in advance.
 
I have never seen an opamp with a GBP of only 100kHz. Most are at least 1MHz. Audio opamps are 3MHz to 8MHz.

Power amplifiers are bridged, not opamps. Please post the schematic of what you want to do.
 
audioguru said:
I have never seen an opamp with a GBP of only 100kHz. Most are at least 1MHz. Audio opamps are 3MHz to 8MHz.

Power amplifiers are bridged, not opamps. Please post the schematic of what you want to do.
Click to expand...

Thanks for the reply,
Well actually I was talking about power amplifiers, for instance GBP for LM3886 is 100kHz.
They seem to be kind of power OP-AMPs I guess.
 
The gain bandwidth product for the LM3886 power amplifier is typically 8MHz, not just 100kHz. It is tested at 100kHz.
You can bridge two of them if you add an offset voltage null trimpot then their power will be 136W into 8 ohms.
 
audioguru said:
I have never seen an opamp with a GBP of only 100kHz. Most are at least 1MHz. Audio opamps are 3MHz to 8MHz.

Power amplifiers are bridged, not opamps. Please post the schematic of what you want to do.
Click to expand...
take a look at micro power opamps....they are often 100KHz or below
 
Micro power opamps are so slow that I never use them. The lousy old 741 opamp has a full power bandwidth of only 9kHz but its GBP is 1MHz.
 
Note that the LM3886 is pretty useless at 8MHz because it has a gain of 1 at that frequency.

I haven't looked at the datasheet but I assume it probably starts to roll off at about 200kHz.
 
audioguru said:
The gain bandwidth product for the LM3886 power amplifier is typically 8MHz, not just 100kHz. It is tested at 100kHz.
You can bridge two of them if you add an offset voltage null trimpot then their power will be 136W into 8 ohms.
Click to expand...

Oh my bad, You are right auduguru, I looked at the datasheets of LM3886 and LM3875 and you are right.
But another question with regard to this point is, if the gain is just 1 at 8MHz for these Audio power amplifiers then they should work just fine at frequencies beyond 20kHz? For instance the gain for LM3886 and LM3875 at 100kHz is 80. So do they work just fine and linear at Bandwidth of 100kHz bu the gain 80 or less? Or there is another factor to be considered?
I mean If in need an power amplifier at higher frequencies (for instance at 100kHz with the gain of 80 or at 1MHz with the gain of 8) in future then those chips are an option to be considered?

Thanks
 
The output of the LM3886 has slew-rate limiting above 100khz at full output. So frequencies above 100kHz at full output will be reduced in level and turned into triangle waves.

If you operate a power amplifier without any negative feedback then distortion is increased.
 
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Soundman, the GBP (or GPB as you call it) is the Gain Bandwidth Product. It is the frequency at which the gain of the amplifier drops to zero gain, or an amplification factor of 1.

If you bridge 2 amps, the GPB of each individual amp does not change, but with one amp driven 180 degrees out of phase, the output across the output terminals of both amps will have double the voltage. The total gain will be 6dB higher.

For reasons of stability, most amplifiers are wired to have a feedback loop roll off at 6dB per octave where the gain drops to zero on the response curve. You'd have to measure the frequency response to be sure. If it does, the GBP of the bridged amp will be one octave higher. The 100KHz GBP becomes 200KHz.
 
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Hi Hero! It is good to see you back.

Hero999 said:
Note that the LM3886 is pretty useless at 8MHz because it has a gain of 1 at that frequency.
I haven't looked at the datasheet but I assume it probably starts to roll off at about 200kHz.
Click to expand...

You would be surprised! A typical unity gain stabilised op amp has a roll-off of 6db/Octave (20dB/decade) and an open loop amplification factor in excess of 100,000. That is an open loop gain of 100dB. Typical old TL074 op-amp has a GBP of 3 MHz. This means that the open loop response starts rolling off near 30Hz. Yup, only 30. No, not KHz. Hz.
 
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When the gain is 100dB, the rolloff begins at 30Hz.
When the gain is 80dB, the rolloff begins at 300Hz.
When the gain is 60dB, the rolloff begins at 3kHz.
When the gain is 40dB, the rolloff begins at 30kHz.
When the gain is 20dB, the rolloff begins at 300kHz.
When the gain is 1, the rolloff begins at 3MHz.

It is done so that the opamp does not oscillate at high frequencies. It doesn't have any gain above 3MHz so it cannot oscillate when negative feedback becomes positive due to phase shift.
 
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