Contents
What does a differential amplifier do?
The differential amplifier circuit amplifies the difference between signals applied to the inputs (Figure 3.5). Superposition is used to calculate the output voltage resulting from each input voltage, then the two output voltages are added to arrive at the final output voltage.
What is a differential input?
Differential inputs measure the voltage between two distinct input signals (see Figure 2). A differential input better resists electromagnetic interference (EMI) than does a single-ended input. The DAQ input measures only the difference between the two leads, and the EMI common to both is ignored.
Why does my phono preamp have low DC?
For my own use that was of no importance, my main preamp has capacitors feeding the volume control, but for anyone needing very low DC at the phono preamp output I should suggest a solution. The cause of the problem is the interaction between channels via the 5k6 resistor (R10) forming part of the ‘rumble filter’.
Which is the best way to short an op-amp?
A better idea is to short one op-amp inverting input to earth and then adjust the other channel preset for zero DC output. Then remove the short and set that channel also. That way both op-amps have their inverting inputs set close to zero, and the outputs are also both close to zero.
Can a pot be adjusted for 0V output?
The pot acts as an ‘offset adjustment’, and the input amplifier can be adjusted for 0V output, or for symmetrical clipping if preferred. Having measured the pot resistance needed it could then be replaced with a fixed resistor.
Why does the op-amp distortion rise with output loading?
The op-amp distortion also rises with output loading, and may include some crossover effects because of the class-B output stage. For these reasons a class-A output buffer transistor is added, with a 220R base-emitter resistor included to take a few mA output current from the op-amp in only one direction and prevent internal class-B switching.