p channel jfet or mosfet (cmos-half-bridge oscillator)

[julian403]

p chanel jfet or mosfet (cmos half-bridge oscillator)

Hello, I'm looking for a fast p chanel jfet or mosfet which is on with a Vgs=0 and goes off with a Vgs>0. I'm looking it because i want fo do CMOS inverter to do an oscillator. I means the transistor must be in satiration state with Vgs=0.

 

[chemelec]

re: p chanel jfet or mosfet (cmos half-bridge oscillator)

It Can't go Both on and Off with Vgs = 0.

 

[julian403]

re: p chanel jfet or mosfet (cmos half-bridge oscillator)

No, what I need is a transistor which a Vgs(th)= 0 (the chanel resistance is lowest if vgs=0) and with a Vgs>0 the chanel resistance go up.



The upper transistor, Q1 must be on firts (there is 0V at the gates), untill C1 where at Vcc potential, Q1 goes off and Q2 goes on and there is an oscilation. But without a negative potential Q1 must be on with ground on the gate.

 

[BradtheRad]

re: p chanel jfet or mosfet (cmos half-bridge oscillator)

Your oscillator circuit is clever. It gets continuous oscillations with only a single half-bridge. Put the Pmos at the top, Nmos at the low side. It gets going quickly.



Frequency over 580 MHz. It may be easier to do this with microwave transistors.

 

[FvM]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

As Brads simulation shows, there's no need to use a depletion type P channel transistor for the circuit. A regular unbuffered CMOS inverter built of complementary enhancement transistors will do. It's a standard circuit e.g. used for IC crystal oscillators.

The schematic in post #3 has a wrongly connected PMOS transistor, please correct.

 

[julian403]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

thanks for the answer. Using a CMOS inverter there is an oscilation



But I want to do it by myself and generate a sine wave. What transistors did you use BradtheRad?

If a put a enhancement-type n-chanel mosfet at top, the chanel's resistance is only low (on state) if there is a positive Vgs voltaje on it. So the capacitors will never have Vcc voltaje on it. I still do not understand how a enhancement n chanel transistor at the top can generate an oscilator with out a positive voltage on the gate, at first .

 

[FvM]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

CD4049BP is a buffered inverter with high gain (three series connected inverters). You better use an unbuffered inverter.

Due to DC feedback, the inverter input is biased to about VCC/2 with all transistors in saturation.

 

[julian403]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

with a comertial CMOS unbuffered inverter is possible to have a 150 MHz sine wave? What is that comertial CMOS inverter?

 

[dick_freebird]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

150MHz needs some pretty short channel devices,
and that means low voltage logic families. Look to
74LVC and so on (there are many variants), about
2.5 or 1.8V is likely to be capable (3.3V SOI, I saw
a lot of edge-rounding at 80MHz even). Now too
fast might make your sine wave more distorted as
harmonics come in, but too slow and you lack the
gain @ freq that you need. There's probably a
"sweet spot". You might dig in at vendors who
offer this voltage range of products, find any
unbuffered-inverter products and see what they
show for xtal osc applications (one of the few
applications where people pick single stage
inverters on purpose).

 

[chemelec]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

Why would you want 150Mhz Sine Wave?
What is your INVERTER being used for?

 

[julian403]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

Ask transmitter. So with an unbugffered cmos inverter i can generate a sine wave.

 

[FvM]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

Fast CMOS gates like 74AHC1GU04 or 74LVC1GU04 might work at 150 MHz.

 

[chemelec]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

Why don't you get a 150Mhz crystal and create a transistor oscillator.
Than it will be Sine wave.

 

[julian403]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

Why don't you get a 150Mhz crystal and create a transistor oscillator.
Than it will be Sine wave.

I thougth with a pierce oscillator it will be more easy but I think i was wrong

 

[dick_freebird]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

I imagine that you would probably put a tuned filter
between the oscillator and any power amplifier to
clean up the harmonics from the inverter-oscillator.

CMOS inverter oscillators are usually meant to be
the head-end of a square wave clock generator.
I don't think people care much about their linearity /
spectral purity in such applications, they just gain
it up until it's square anyway.

So if you care about what comes out your antenna
you may want either a cleaner form of oscillator, or
to clean up this one post-facto.

 

[BradtheRad]

Re: p chanel jfet or mosfet (cmos half-bridge oscillator)

What transistors did you use BradtheRad?

If a put a enhancement-type n-chanel mosfet at top, the chanel's resistance is only low (on state) if there is a positive Vgs voltaje on it. So the capacitors will never have Vcc voltaje on it. I still do not understand how a enhancement n chanel transistor at the top can generate an oscilator with out a positive voltage on the gate, at first .

The arrangement of PNP & NPN biases both of them immediately on power-up. This encourages them to conduct. Certain conditions need to be right, for oscillations to begin.
(Although there are capacitors in the current path, apparently they allow sufficient current flow before blocking DC current, so that oscillations begin.) Some degree of imbalance is often necessary as power is applied, or an oscillator may fail to get going.

I've been working with similar circuits as yours, using real components. Ordinary general purpose BJT (2SA933, 2SC1740). Various values of inductors & capacitors.
Under 1 MHz (due to limitation of my oscilloscope and/or components).

I find that certain factors make a difference whether oscillations sustain or not:

* L:C ratio

* supply voltage (Often confined to a narrow range.)

* bias resistor values

* characteristics within the inductor (besides Henry value & ohmic resistance) which I'm not sure how to quantify nor measure.