[alley-kat]

hi there, could someone please explain to me what points to measure in the class a bias cascode circuit. i took one measurement from source and gate tied together to pin 4-v. and i had one miliamp. is this how to measure this? and do i need to take any other measurements? i am using 2 2n3819s. thanks alley-kat

 

[tangent]

I guess that depends on what you're trying to measure....

If you want to measure the current through the cascode, you break the circuit above or below the cascode and put the milliammeter across the break.

 

[alley-kat]

hi tangent, what i'm needing to find out is if the q-1 transistor has less idss in the circuit than q-2. if i understand correctly, a resistor is added when using two transistors of the same kind, to reduce the current through q-1. is this correct? if so, how can i verify that? i guess thats what i'm really trying to do. thanks, alley-kat

 

[tangent]

If you're using two JFETs of a type in a cascode, it's best to measure them outside the application circuit. That way, you can simply pick Q2 to be one of the higher-Idss ones.

If your JFETs are already planted and you don't know which one has the higher Idss, break the circuit above or below the cascode, put the milliammeter across the break, and then vary the supply voltage and observe how current changes as a percentage. Add varying source resistors to the lower JFET until you get the best balance between current draw and regulation.

In the future, if you want the convenience of being able to solder down random JFETs (as opposed to picking suitable sets from a big batch), use different types of JFETs for Q1 and Q2. Make Q2 a type with a higher minimum Idss than Q1's maximum Idss. As long as Q1's actual Idss is well above your target current draw, you can simply adjust the source resistor on Q1 to get the current level you want through the whole cascode. This requires that you have a wide selection of resistors, but that's a good thing to have in any case. Buying dozens of JFETs when you only want 4 is less useful, I think.

 

[alley-kat]

hello tangent, thanks for the great reply just what i needed. i have checked a 2sk170 and a 2n3819 using the 9volt and breadboard approach. the 2sk170 read about half what the 2n3819 did so, i asume i would use the 3819 as Q-2 and the 170 as Q-1? also, is this the full max current i am reading according to the voltage applied? without source resistor, between source, and pin 4-v. i had about 3.84 milliamps current so, i added enough resistance for 1 milliamp. is the the correct approach? thanks very much, alley-kat

 

[tangent]

Quote:

i asume i would use the 3819 as Q-2 and the 170 as Q-1?

Yes.

Quote:

is this the full max current i am reading according to the voltage applied?

Voltage doesn't enter into it. That's the point of a current source: current stays the same as voltage changes. A cascode isn't an ideal current source (nothing's ideal) but it's close.

The total current draw you get through the cascode will be less than Q1's current draw alone. Without a source resistor, the cascode's current draw will be roughly half the draw of Q1 alone, and as you increase the resistance between Q1's gate and source, the cascode's draw falls.

Quote:

i had about 3.84 milliamps current so, i added enough resistance for 1 milliamp. is the the correct approach?

Yes.

 

[alley-kat]

hi tangent, preamp sounds great now. thanks for all your help alley-kat