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Regarding this circuit I found on www.geofex.com detailing the famous Tube Screamer guitar pedal

enter image description here
(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)

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2 Answers 2

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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.

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  • \$\begingroup\$ What kind of BJT is providing that kind of input impedance? Or is it the \$ \beta \$ multiplying effect doing that work? I'm a little more used to seeing a JFET there. \$\endgroup\$
    – Daniel
    Apr 25, 2017 at 14:43
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    \$\begingroup\$ @Daniel the emitter load is maybe 5 kohm so at the base this will look like hundreds of kohm. \$\endgroup\$
    – Andy aka
    Apr 25, 2017 at 14:54
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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.

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  • \$\begingroup\$ If I connected a 510K resistor instead of the 10K? \$\endgroup\$ Jul 11, 2014 at 13:11
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    \$\begingroup\$ I mean would the circuit still work the same? Is there any advantage in using the 10K resistor there before the op amp that validates the requirement to buffer the input first? \$\endgroup\$ Jul 11, 2014 at 13:14

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