Hi The following diagram is a circuit from a synthesizer book by R.A Penfold.
The text that describes the circuit is as follows. My question is can anyone please explain to me what the diode does and why it seems to be shorted out across the 741 opamp.
Here is the text that describes the circuit (Copied from the book):
From every log opamp I have seen the diode is part of the feedback of the opamp. But here it is a very strange set up to me. I have no idea what is going on.
This converter circuit, in common with most other types of logarithmic amplifier, relies for its operation on the fact that the current through a forward biased silicon diode rises exponentially with linear increments [it!] the input voltage. Although this characteristic is only maintained over certain limits, in this application there is no need to exceed these limits as only a modest ,range of output currents is involved (no more than a range of about 100 to 1). What does complicate things is that the voltage across a forward biased silicon diode varies significantly with changes in temperature, and diodes are often used as electronic temperature sensors. The circuit must therefore include temperature compensation to avoid the need for very frequent readjustment to correct tuning drift.
Having tried a variety of configurations, this one seems to give reasonably accurate and stable results without the need for any “difficult to obtain” components. IC2 is a CA3046 transistor array, which consists of three individual transistors plus two others connected as a long-tailed pair (i.e. having their emitters connected together). In this circuit only two of the individual transistors are used, and no connections are made to the other devices. IC2a operates as the converter, while IC2b provides a degree of temperature compensation. The point of using a transistor array to provide these two devices is that this ensures excellent thermal contact between the two, and consequently gives instant and accurate temperature compensation. The alternative of using two ordinary silicon NPN transistors with their cases glued together seems to work reasonably well, but the additional expense of using a transistor array is probably justified.
The input voltage range is far too large to directly drive IC2a, and a potential divider to provide a suitable degree of attenuation is therefore included at the input of the circuit. RV1 is adjusted to give the required 1 volt per octave characteristic. RV2 is the frequency control, and this enables the output current of the circuit to be adjusted. In practice this acts as the tuning control, and it is adjusted to give the required pitch range from the VCO. It provides a large control range, and enables the pitch range to be shifted over at least three octaves.
The text that describes the circuit is as follows. My question is can anyone please explain to me what the diode does and why it seems to be shorted out across the 741 opamp.
Here is the text that describes the circuit (Copied from the book):
From every log opamp I have seen the diode is part of the feedback of the opamp. But here it is a very strange set up to me. I have no idea what is going on.
This converter circuit, in common with most other types of logarithmic amplifier, relies for its operation on the fact that the current through a forward biased silicon diode rises exponentially with linear increments [it!] the input voltage. Although this characteristic is only maintained over certain limits, in this application there is no need to exceed these limits as only a modest ,range of output currents is involved (no more than a range of about 100 to 1). What does complicate things is that the voltage across a forward biased silicon diode varies significantly with changes in temperature, and diodes are often used as electronic temperature sensors. The circuit must therefore include temperature compensation to avoid the need for very frequent readjustment to correct tuning drift.
Having tried a variety of configurations, this one seems to give reasonably accurate and stable results without the need for any “difficult to obtain” components. IC2 is a CA3046 transistor array, which consists of three individual transistors plus two others connected as a long-tailed pair (i.e. having their emitters connected together). In this circuit only two of the individual transistors are used, and no connections are made to the other devices. IC2a operates as the converter, while IC2b provides a degree of temperature compensation. The point of using a transistor array to provide these two devices is that this ensures excellent thermal contact between the two, and consequently gives instant and accurate temperature compensation. The alternative of using two ordinary silicon NPN transistors with their cases glued together seems to work reasonably well, but the additional expense of using a transistor array is probably justified.
The input voltage range is far too large to directly drive IC2a, and a potential divider to provide a suitable degree of attenuation is therefore included at the input of the circuit. RV1 is adjusted to give the required 1 volt per octave characteristic. RV2 is the frequency control, and this enables the output current of the circuit to be adjusted. In practice this acts as the tuning control, and it is adjusted to give the required pitch range from the VCO. It provides a large control range, and enables the pitch range to be shifted over at least three octaves.
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