FOR THE IMPATIENT: Skip to Point 6, the formula.

1. Just as with BJTs, we must account for the internal resistance of the JFET source. For the JFET, this resistance is 1/Gm (Gm in Siemens or mhos). (For BJTs, use Shockley’s Constant, 26/Ic, in mA. The answer to this equation is in series with the external emitter resistor.)

2. Point #1 suggests a revised formula for JFETs: Vg= (Rd || Rl) / (Rs + Rs_internal). Yet life isn’t this simple.

3. We must also account for the source capacitor Cs. Two problems here: (A) The source cap does not bypass the internal source resistor Rs_internal. (B) Despite the argument that Cs “eliminates Rs,” Cs does not do that. In fact, Cs: (C) Eliminates the JFET’s DC amplification capability. (D) Renders the JFET nonlinear, by introducing a high-pass response curve.

4. To account for the high-pass curve, we must select the frequency where we want to measure Vg. (Typically 1 KHz.) For example, the Vg at 100 Hz will be much smaller than the Vg at 1 kHz. The Vg at DC will be about zero. (DC-Vg depends on the leakage resistance of Rs.)

5. We can reduce, but not eliminate the high-pass effect by increasing the Cs value. The ideal value will be expensive. But it won’t eliminate the problem.

6. Proposed new Vg formula: [(Rd || Rl) / ((Csx || Rs) + 1/Gm)], where Csx is the capacitive reactance of Cs, in ohms, at the frequency of Vg measurement. Or: Use 10% of The R2 value, in ohms.

7. There are other factors that affect Vg. For example, I’m ignoring (A) Internal capacitances. (B) RCL of circuit wiring. (C) Typical variability over a 5X range of name-brand JFET parameters. (D) Gm measurement at a different Id/Vg curve than what the subject circuit is using. (E) The unknown-unknowns.

FOR THE IMPATIENT: Skip to Point 6, the formula.

1. Just as with BJTs, we must account for the internal resistance of the JFET source. For the JFET, this resistance is 1/Gm (Gm in Siemens or mhos). (For BJTs, use Shockley’s Constant, 26/Ic, in mA. The answer to this equation is in series with the external emitter resistor.)

2. Point #1 suggests a revised formula for JFETs: Vg= (Rd || Rl) / (Rs + Rs_internal). Yet life isn’t this simple.

3. We must also account for the source capacitor Cs. Two problems here: (A) The source cap does not bypass the internal source resistor Rs_internal. (B) Despite the argument that Cs “eliminates Rs,” Cs does not do that. In fact, Cs: (C) Eliminates the JFET’s DC amplification capability. (D) Renders the JFET nonlinear, by introducing a high-pass response curve.

4. To account for the high-pass curve, we must select the frequency where we want to measure Vg. (Typically 1 KHz.) For example, the Vg at 100 Hz will be much smaller than the Vg at 1 kHz. The Vg at DC will be about zero. (DC-Vg depends on the leakage resistance of Rs.)

5. We can reduce, but not eliminate the high-pass effect by increasing the Cs value. The ideal value will be expensive. But it won’t eliminate the problem.

6. Proposed new Vg formula: [(Rd || Rl) / ((Csx || Rs) + 1/Gm)], where Csx is the capacitive reactance of Cs, in ohms, at the frequency of Vg measurement. Or: Use 10% of The R2 value, in ohms.

7. There are other factors that affect Vg. For example, I’m ignoring (A) Internal capacitances. (B) RCL of circuit wiring. (C) Typical variability over a 5X range of name-brand JFET parameters. (D) Gm measurement at a different Id/Vg curve than what the subject circuit is using. (E) The unknown-unknowns.

8. If we don’t know the value of 1/Gm: Use the value of Rs, in ohms. (See Albert Malvino, Transistor Circuit Approximations, 3rd ed.)