Hi, I'm back after two cataract surgeries and related post-surgery problems....
I would like to know, in a preamp, if there are any differences between a buffered scheme and one with unity gain.
Thanks!!
I would like to know, in a preamp, if there are any differences between a buffered scheme and one with unity gain.
Thanks!!
Yes, not necessarily the same, but one stage could have both attributes.
Buffer stages provide lower output impedance and higher current, than without the buffer stage.
Often the buffer stage will have relatively high input impedance to avoid loading the previous stage.
So the main purpose is to reduce interaction between the previous stage, and the following stage.
Sometimes the buffer stage has unity gain, but not necessarily. It can both buffer and have voltage gain.
But a cathode/source follower buffer stage will always have less than unity gain.
Unity gain just means that the stage voltage gain = 0 dB ( x1 gain factor). No spec implied for the output impedance
or the current capability.
Buffer stages provide lower output impedance and higher current, than without the buffer stage.
Often the buffer stage will have relatively high input impedance to avoid loading the previous stage.
So the main purpose is to reduce interaction between the previous stage, and the following stage.
Sometimes the buffer stage has unity gain, but not necessarily. It can both buffer and have voltage gain.
But a cathode/source follower buffer stage will always have less than unity gain.
Unity gain just means that the stage voltage gain = 0 dB ( x1 gain factor). No spec implied for the output impedance
or the current capability.
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Thank you for your fast reply.
Consider this schematic. so is it better to leave it like this or put a buffer instead of unity gain?
Consider this schematic. so is it better to leave it like this or put a buffer instead of unity gain?
Firstly it is good that the operations are over. I wish you fast recovery.
It depends which one of the 2 x OP27 you mean and they're both not unity gain configured. If you replace the 10 kOhm for a wire at the first OP27 and remove the 10 kOhm from pin 2 to GND you'll have the (input) buffer. Same counts for the second opamp but then you have no gain anymore. I learnt that a buffer has the properties as described by rayma but with 0.99x gain. In short a gain of 1x. In most applications with normal 2Vrms sources you'll need very little gain and 2x or 3x is more than enough otherwise things might get nervous with normal gain power amplifiers like too high volume at 10 o'clock. It is better to add an input cap at the signal inputs in case the source has DC offset but beware for sources that already have an output cap -> 2 caps in series!
If you have normal 2Vrms sources it is even better to omit the first opamp alltogether, leave the second OP27 with a gain of 3 and just add an input cap and 100 kHz RC lowpass filter before the potentiometer. Filtering input signals is a good habit. Check your sources if they're DC coupled, it will make clear if you need input caps or not. A slight DC offset (that will be amplified 2x by the first opamp in the original configuration) can be enough to make the potentiometer scratch annoyingly.
It depends which one of the 2 x OP27 you mean and they're both not unity gain configured. If you replace the 10 kOhm for a wire at the first OP27 and remove the 10 kOhm from pin 2 to GND you'll have the (input) buffer. Same counts for the second opamp but then you have no gain anymore. I learnt that a buffer has the properties as described by rayma but with 0.99x gain. In short a gain of 1x. In most applications with normal 2Vrms sources you'll need very little gain and 2x or 3x is more than enough otherwise things might get nervous with normal gain power amplifiers like too high volume at 10 o'clock. It is better to add an input cap at the signal inputs in case the source has DC offset but beware for sources that already have an output cap -> 2 caps in series!
If you have normal 2Vrms sources it is even better to omit the first opamp alltogether, leave the second OP27 with a gain of 3 and just add an input cap and 100 kHz RC lowpass filter before the potentiometer. Filtering input signals is a good habit. Check your sources if they're DC coupled, it will make clear if you need input caps or not. A slight DC offset (that will be amplified 2x by the first opamp in the original configuration) can be enough to make the potentiometer scratch annoyingly.
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With op amps, things are a little different. The op amp (+) input has relatively high input impedance,
and the op amp output has relatively low output impedance. So it acts as a buffer itself in many cases.
Most op amps can drive a 2k load well enough without taking any further measures.
The first stage has a voltage gain of x2 (+6dB), and the second stage has a gain of x3 (+9.5dB).
However I would reduce the 1M input resistor to a lower value, depending on the source, and
the value of any DC blocking capacitor present in the source.
You may be getting at gain structure considerations. For this you also need to know the output voltage of the source,
and the power amplifier input sensitivity. The idea is to drive the power amplifier to full output, without clipping any
of the previous stages, and without excessive noise.
http://www.learningaboutelectronics.com/Articles/Op-amp-gain-calculator.php
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