I would like to use a JFET, like a J113, as an audio switch that is in series with the signal. The audio input signal is tied to the Drain, and the audio output signal is tied to Source. The audio signal fluctuates above and below 0V. The Gate is controlled by a control voltage, where 0V= switch is on, and -5V= switch is off. I have seen circuits where a diode is placed between the control voltage and the Gate. The anode of the diode is tied to the Gate and the cathode is tied to the control voltage.
Why is this put there? In describing this switch circuit, Kevin O'Connor in the book "The Ultimate Tone" writes on page 9-12, "A reverse-biased diode in series with the j-fet gate allows the j-fet to find its own "on-state." Does anybody know what he is talking about?
I understand that the Source-Gate voltage difference determines 'on resistance' of the jfet and the current that can pass through the jfet. Since the audio signal is going through the Source, there will be some distortion as the Source fluctuates in relation to the steady 0V at the Gate. Does Gate diode help with this? I also don't know why he calls it reverse-biased. Is that because it points in the opposite direction as the arrow on the jfet symbol?
It seems that a reverse-biased diode on the Gate lets the current flow to -5V when the control voltage is -5V, but when the control voltage is brought to 0V, it seems that the electrons at the Gate would be trapped there by the diode. My breadboard experiments show that this is not the case. The switch turns on and off. However, it is slower to turn on than off. Also, if I switch the diode around - i.e. cathode tied to the Gate - the switch turns on fast and off more slowly. Is there some diode leakage, in the micro or nano amp amounts, that is enough to change the voltage on the Gate?
How, does the diode's voltage drop fit into this, also? I have tried to measure this with my multi-meter, but I don't seem to be measuring any voltage drop between the anode and cathode. Only the cathode is tied to a voltage source, the anode/Gate side is floating. It seems to me that without the diode being tied to voltages on both sides, its behavior would be random (static electricity fluctuations).
I am sorry if I am not able to write more coherent questions. I just don't get what the diode does for the simple JFET audio switch circuit.
Thanks for any thoughts,
Dave
Why is this put there? In describing this switch circuit, Kevin O'Connor in the book "The Ultimate Tone" writes on page 9-12, "A reverse-biased diode in series with the j-fet gate allows the j-fet to find its own "on-state." Does anybody know what he is talking about?
I understand that the Source-Gate voltage difference determines 'on resistance' of the jfet and the current that can pass through the jfet. Since the audio signal is going through the Source, there will be some distortion as the Source fluctuates in relation to the steady 0V at the Gate. Does Gate diode help with this? I also don't know why he calls it reverse-biased. Is that because it points in the opposite direction as the arrow on the jfet symbol?
It seems that a reverse-biased diode on the Gate lets the current flow to -5V when the control voltage is -5V, but when the control voltage is brought to 0V, it seems that the electrons at the Gate would be trapped there by the diode. My breadboard experiments show that this is not the case. The switch turns on and off. However, it is slower to turn on than off. Also, if I switch the diode around - i.e. cathode tied to the Gate - the switch turns on fast and off more slowly. Is there some diode leakage, in the micro or nano amp amounts, that is enough to change the voltage on the Gate?
How, does the diode's voltage drop fit into this, also? I have tried to measure this with my multi-meter, but I don't seem to be measuring any voltage drop between the anode and cathode. Only the cathode is tied to a voltage source, the anode/Gate side is floating. It seems to me that without the diode being tied to voltages on both sides, its behavior would be random (static electricity fluctuations).
I am sorry if I am not able to write more coherent questions. I just don't get what the diode does for the simple JFET audio switch circuit.
Thanks for any thoughts,
Dave
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