Mosfet Switched capacitor in LC tank

Question

I need to coarse control the frequency of oscillation on a parallel resonant tank. The frequency band is in the 500kHz-1Mhz range, so the small capacitance of varactors doesn't suit the task.

Another way of doing it is by switching a capacitor branch with a MOSFET

^{simulate this circuit – Schematic created using CircuitLab}

The problem is that the tank is floating, thus the source is not fixed biased and that makes it difficult to control the conduction on the MOSFET.

I've not considered adding a PLL or another stage because it's the coil where I need the output to be and the current in the tank is already pretty high.

A second problem is that even if I can switch the capacitor on and off, the amplitude of oscillation will vary according to the impedance change, and I need to get as stable amplitude as possible.

¿Are there any solutions for this specific arrangement or is it better to use a grounded tank so I can switch the then properly biased MOSFET?

Thanks in advance!

• Note the values in the schematic are somewhat random

Answer 1

I see two problems with that circuit:

1) as you wrote: the biasing of the MOSFET

2) The bulk diode of the MOSFET, when on the tank the voltage on the top node is negative relative to the bottom node, the source-bulk diode in the NMOS will turn on. Oops !

I have a proposal to solve both issues:

^{simulate this circuit – Schematic created using CircuitLab}

The 1 M resistors bias the NMOSFET's Drains to ground, that solves 1)

You cannot get rid of the source-body diode in discrete MOSFETs so I propose to use two in anti-series. Combined the two NMOS will behave as one NMOS without a body diode.

Instead of biasing the drains to ground you can bias them to any DC voltage as long as Vgs is changed accordingly. Such that there's a positive Vgs to turn the NMOS on and Vgs = 0 V to turn them off.

Note that this only allows you to switch on/off capacitance (C2 in series with C4) so the adjustment is coarse but you describe that in your question so you're aware of that.

If you want finer adjustment, place many of these sections with different values for C2 and C4 in parallel with the tank.