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Regarding this circuit I found on www.geofex.com detailing the famous Tube Screamer guitar pedal
(source: geofex.com)
I don't understand the purpose of the BJT input emitter follower transistor. If it was to increase the input resistance of the pedal then the TL072 would have much greater input impedance than a BJT no? (I read approximately 10^12 in the datasheet for TL07x)
The TL072 may have a high input impedance but it's also got significant input bias currents and if the front BJT was done away with, the leakage current would amass a voltage across the 510 kohm input resistor of 3.57 milli volts.
What, a paltry 3.57 milli volts you might say?
You've got to remember that a guitarist might be playing very quietly to get a smooth distortion sound and an amplitude of a few milli volts isn't unheard of - that's point 1. Point 2 is that the 3mV offset might push the op-amp to bias one of its feedback diodes more than the other and suddenly you have a signal gate - the input signal has to overcome the blockage caused by the bias current into a large value resistor.
BJTs are far more robust against careless musicians too.
No. In the circuit configuration shown, the input impedance of the first opamp stage is dominated by the values of the resistors, such as the 10K tied to +4.5V. The transistor isolates the source from this load.
