voltage controlled resistor

Hi everyone,

I'm new on this forum, and i'm hoping for your help!

I'm currently working on the design of an ultrasonic sensor. I'm using a 40kHz ultrasonic transceiver (both receive and transmit). The transceiver is driven by a transformer with 1:10 turn ratio. Parallel to the transducer i want to implement a variable resistor with high resistance at the time of the ultrasonic burst (50kohm-100kohm) and low resistance just after the burst (500ohm - 1kohm) for probably around 0,1ms. Then the resistance should be high again in order for the received echo not to be damped.

I red quite a lot already about jfets as voltage controlled resistors. Still i'm finding it hard to find the right one. The drain will be grounded, the source will be an ac-signal (+30 volt to -30 volts) and the gate will be max 3,3 volts.

Do you guys have any ideas on which jfet to use?

rbolijn said:

The drain will be grounded, the source will be an ac-signal (+30 volt to -30 volts) and the gate will be max 3,3 volts.

Click to expand...

This will not work. Instead, the source must be grounded. Only in this case the resistance determining gate-source voltage Vgs can be kept constant (independent on the ac signal).

LvW said:

This will not work. Instead, the source must be grounded. Only in this case the resistance determining gate-source voltage Vgs can be kept constant (independent on the ac signal).

Click to expand...

Off course, you're right. My mistake.

Still, can this work with an ac-signal of (30 V+, 30 V-)? How should i choose the jfet? Most important characteristic should be the possibility for low resistance of 500ohm/1kohm and high resistance (couple hundreds kohms). High resistance could be lower, but then i should put an extra resistor in series. Would the extra resistor have an effect on the working of the jfet resistance?

Thanks for your help

To switch of a FET in presence of +/-30 V AC voltage, you need to apply at least - 30V to the gate, otherwise Vgd will be forward biased during negative halfwave. The transistor needs a gate breakdown voltage > 60V, which isn't available, as far as I know. 2N38xx types have about 50V.

It would be more realistic to use a bipolar switch with a fixed series resistor. But the switch (e.g. antiserial circuit of two MOSFET) needs a floating gate drive circuit, involving a rather high effort. I would refer to a small signal solid state relays like Matsushita AQY221N2V for similar applications. https://www.panasonic-electric-works.de/catalogues/downloads/photomos/ds_x615_en_aqy221__v.pdf

As a final comment, most designers are solving the problem of transducer damping with a fixed resistor, although you need a higher drive power. But the sending waveform is better controlled and a better echo time resolution can be expected. Simply assuming impedance matching, also the receiver gain has to be doubled. All in all it's less effort than the switched transducer damping, I think.

Having an output transformer may help you: place your jfet or cmos switch on the primary side.
Both the voltages and the impedances involved are lowered this way.

Having an output transformer may help you

Click to expand...

Your question raises the question of involved driver impedance. If the transformer primary is driven by voltage source, a switched resistor at the primary would be useless and the transducer is already shorted by the transformer leakage inductance. If it's a kind of tri-state or open collector driver, it can work. Leakage inductance is however changing the load impedance.

I am posting this to fill in for one or two that were lost (including my post and a reply). They must have been casualties of a hacker attack. I copied the following from my routine email notifications of new posts in subscribed threads.

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I attached the transmit circuit to get better insight.

---Quote (Originally by BradtheRad)---
Your OP speaks of wanting to vary the resistance of one component. However if the purpose is to make it alternate between two values, then it might work just as well to use fixed resistors. The switching device is the challenge. It must allow electrons to flow both ways.
---End Quote---

You're right, maybe the problem wasn't clear: i want to switch between two different resistor values.

So, the primary side is driven by a voltage source so switching at that side doesnt work according to FvM. That solid state swithes (AQY221N2V) are too expensive for my purpose.

@BradtheRad: Load switches are normally for either high or low side right? Not for bidirectional purposes? The 4066 look usable though (i'll look for higher voltage rating)! This ic uses the technique of 2 mosfet back to back right? What are these ic's called? Are there any cons to using such a technique?

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BradtheRad's reply to above:

Yes the 4066 contains 2 mosfets. The datasheet claims that it can handle analog signals, and that the input/output pins are reversible. It's one of those IC's that one wonders whether it can really live up to the claims. It has worked for me in at least one project. It may or may not work for your application.

There are load switches of various kinds. I was thinking of a solid-state type I saw recently whose schematic resembles a beefed-up 4066. One of those may handle the range of voltages you are using (positive and negative).

Depending on the voltage level over which the VCR must be linear, the Fairchild H11F1M might work.

This is a photofet which changes from 300M ohm to about 100 ohms in 25usec. The LED requires 10mA - 16mA.