Hi all.
I've recently been tinkering with jfet drives, in particular using a 'sag' control to change the voltages at the drain of the SMD J201s that I'm using - much like the Fairfield Barbershop. In broad terms, the circuit I'm working on comprises a buffer stage followed by two gain stages.
I have a B22k pot connecting the 9v supply to the resistors feeding Q2 and Q3, which are 22k and 6.8k respectively. My question is this. Why when I come to measure the voltages at the drain of Q2 and Q3 do they measure exactly the same as one another regardless of where the 'sag' control is set? For example, at minimum sag the voltages both measure around 7.8v and at maximum sag the voltages both measure around 2.5v.
My expectation was that, given that the 22k resistor connected to the drain of Q2 is over three times the value of the 6.8k resistor connected to the drain of Q3, they would yeild different voltage measurements. Am I wrong here for some reason? If somebody could explain to me what might be going on I'd greatly appreciate it!
I've attached the schematic for the Fairfield Barbershop as a reference. While the values are different the principle is the same.
(https://i.postimg.cc/mzDg2hDw/barbershop-v2-schematic.png) (https://postimg.cc/mzDg2hDw)
What are the values of Source resistors..??
In case of similar Rd/Rs ratios, your measurements are right.. :icon_wink:
Hi Antonis.
On Q2 I have a B10k pot connected - lug 3 to source, lug 1 to ground and lug 2 to a 470n cap going to ground.
On Q3 connected to the source I have a 1k5 resistor and a 22uf cap both going to ground.
That could make sense for VGS and IDSS spread..
BTW, don't you aim for equal Drain bias..??
Quote from: Greenballs on March 20, 2025, 09:43:53 AMHi all.
I've recently been tinkering with jfet drives, in particular using a 'sag' control to change the voltages at the drain of the SMD J201s that I'm using - much like the Fairfield Barbershop. In broad terms, the circuit I'm working on comprises a buffer stage followed by two gain stages.
I have a B22k pot connecting the 9v supply to the resistors feeding Q2 and Q3, which are 22k and 6.8k respectively. My question is this. Why when I come to measure the voltages at the drain of Q2 and Q3 do they measure exactly the same as one another regardless of where the 'sag' control is set? For example, at minimum sag the voltages both measure around 7.8v and at maximum sag the voltages both measure around 2.5v.
My expectation was that, given that the 22k resistor connected to the drain of Q2 is over three times the value of the 6.8k resistor connected to the drain of Q3, they would yeild different voltage measurements. Am I wrong here for some reason? If somebody could explain to me what might be going on I'd greatly appreciate it!
I've attached the schematic for the Fairfield Barbershop as a reference. While the values are different the principle is the same.
(https://i.postimg.cc/mzDg2hDw/barbershop-v2-schematic.png) (https://postimg.cc/mzDg2hDw)
So up front: the drain resistor are set so the drain voltages are both 6V. The different *resistor* values account for the different JFET parameters.
Once those resistors are set for each JFET the voltage across each drain resistor is 3V. What's different is the JFETs are drawing different drain currents. in your example Id1 = 3V / 6.8k = 441uA, Id2 = 3V/22k = 136uA.
The key point is the JFETs are acting as constant current devices (more or less). Once the drain resistors are set they will always have 3V drop across them regardless of the sag control. When the sag control resistance is increased the voltage at the top of the drain resistors is reduced but the drain voltages will be 3V below that. So the drain voltages track regardless of the sag control.
Quote from: antonis on March 20, 2025, 02:29:53 PMThat could make sense for VGS and IDSS spread..
BTW, don't you aim for equal Drain bias..??
I arrived at the bias for Q2 and Q3 by using 50k pots and measuring the resistance once I'd settled on the voltages that produced the required results. If I recall correctly, I think Q2 at 22k measured around 4v and Q3 at 6k8 measured around 6.5v? I added the 22k sag control afterwards thinking that this 2.5v difference would be maintained across the sag control's sweep.
But in answer to your question, not always.
Quote from: Rob Strand on March 20, 2025, 05:19:31 PMQuote from: Greenballs on March 20, 2025, 09:43:53 AMHi all.
I've recently been tinkering with jfet drives, in particular using a 'sag' control to change the voltages at the drain of the SMD J201s that I'm using - much like the Fairfield Barbershop. In broad terms, the circuit I'm working on comprises a buffer stage followed by two gain stages.
I have a B22k pot connecting the 9v supply to the resistors feeding Q2 and Q3, which are 22k and 6.8k respectively. My question is this. Why when I come to measure the voltages at the drain of Q2 and Q3 do they measure exactly the same as one another regardless of where the 'sag' control is set? For example, at minimum sag the voltages both measure around 7.8v and at maximum sag the voltages both measure around 2.5v.
My expectation was that, given that the 22k resistor connected to the drain of Q2 is over three times the value of the 6.8k resistor connected to the drain of Q3, they would yeild different voltage measurements. Am I wrong here for some reason? If somebody could explain to me what might be going on I'd greatly appreciate it!
I've attached the schematic for the Fairfield Barbershop as a reference. While the values are different the principle is the same.
(https://i.postimg.cc/mzDg2hDw/barbershop-v2-schematic.png) (https://postimg.cc/mzDg2hDw)
So up front: the drain resistor are set so the drain voltages are both 6V. The different *resistor* values account for the different JFET parameters.
Once those resistors are set for each JFET the voltage across each drain resistor is 3V. What's different is the JFETs are drawing different drain currents. in your example Id1 = 3V / 6.8k = 441uA, Id2 = 3V/22k = 136uA.
The key point is the JFETs are acting as constant current devices (more or less). Once the drain resistors are set they will always have 3V drop across them regardless of the sag control. When the sag control resistance is increased the voltage at the top of the drain resistors is reduced but the drain voltages will be 3V below that. So the drain voltages track regardless of the sag control.
Thanks for your detailed reply Rob, much appreciated. I'm going to take a little time to digest what you've written. I will probably be back with more questions!