At the university we have seen this circuit as a Phase Modulator:


simulate this circuit – Schematic created using CircuitLab

where \$m(t)\$ is the message at audio frequency and \$v_p(t)\$ is the carrier at radio frequency.

The idea is that \$m(t)\$ change the value of \$g_m\$, modulating the phase of \$v_p(t)\$.

The proposed small signal linear circuit is this:


simulate this circuit

The first thing I do not understand is how the bypass capacity for \$ m (t)\$ is a short-circuit at audio frequencies and an open-circuit at radio frequency, as can be seen from the proposed small signal scheme. Also why do you think the prof used this model for JFET? It is an extremely approximate model for very low frequencies, it is not good especially if I work at slightly higher carrying frequencies.

\$g_m\$ depends on \$ m(t) \$, so it is a function of time. In the frequency domain then \$ g_m \$ is a function of \$ \omega \$ and i cannot consider it as a number, which has been done both on the book and the prof, obtaining the following expression (approx):

$$\frac{v_{out}\left( \omega \right)}{v_{p}\left( \omega \right)}=\frac{1-\frac{g_{m}}{j\omega {C}}}{1+\frac{g_{m}}{j\omega {C}}} $$

How can this analysis be correct even if in this case \$ g_m=g_m(t) \$?

Thank you.

PS: Sorry if my English is not very well.

  • \$\begingroup\$ You might want to fix the formulas which didn't get formatted properly. \$\endgroup\$ Jun 28, 2017 at 13:10
  • \$\begingroup\$ How can i insert an inline formula? \$\endgroup\$
    – user153389
    Jun 28, 2017 at 13:12
  • \$\begingroup\$ The proposal doesn't make sense to me. \$\endgroup\$
    – Andy aka
    Jun 28, 2017 at 13:16

1 Answer 1


Capacitor C4 must be in series with inductor L2, this is how the RF signal is blocked.

That is, the polarization of the gate of the FET and the modulating signal enter through L2. Accompanying photo taken from the ARRL Handbook.



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