Types of MOS tubes in Wien bridge

S3S4S5S6

Types of MOS tubes in Wien bridge [Copy link]

 

For this voltage stabilizing circuit made of MOS tubes, can only N-type MOS tubes be used?

maychang

In the picture posted by the OP, the field effect transistor does not seem to be a MOS, but a JFET, that is, a junction field effect transistor.

S3S4S5S6

maychang posted on 2020-7-21 11:56 In the picture of the original poster, the field effect tube does not seem to be MOS, but JFET, that is, junction field effect tube.

"It is a JFET, that is, a junction field effect transistor." Can this tube be replaced with a MOS tube? For example, a 2N7002 type

maychang

S3S4S5S6 Published on 2020-7-21 12:16 "It is a JFET, that is, a junction field effect transistor." Can this tube be replaced with a MOS tube? For example, the type of 2N7002

In principle it is possible, but other parts of the circuit must be modified.

Actually, there may be an error in the diagram on the first floor. Look at the orientation of diode D1.

S3S4S5S6

maychang posted on 2020-7-21 12:36 In principle, it is possible, but other parts of the circuit must be modified. In fact, the first floor diagram may be wrong. Look at the direction of diode D1.

I personally understand that there is no problem with this diagram. The D1 diode should be used to block the forward voltage, because the G-pole control voltage of the JFET is negative.

maychang

S3S4S5S6 Published on 2020-7-21 13:09 I personally understand that there is no problem with this diagram. The D1 diode should be used to block the forward voltage because the G-pole control voltage of the JFET is negative

That's right. The larger the output signal amplitude of the upper op amp, the larger the signal amplitude output by the lower op amp, and the larger the negative DC voltage after rectification by D1. The more negative the gate of the field effect tube, the larger the equivalent resistance, the larger the negative feedback of the upper op amp, and the smaller the voltage gain of the upper op amp.

But the positive pole of D1 is connected to the upper end of C4, and the voltage across C4 should be positive at the top and negative at the bottom. But the upper end of C4 is marked as the positive end of the electrolytic capacitor, and the electrolytic capacitor C4 is connected in reverse.

gmchen

The original poster posted two posts in succession, and the target was an oscillator.

Regarding this Wien bridge oscillator, using a field-effect transistor for gain control can produce a sine wave with very low distortion, but the prerequisite is that the field-effect transistor has good linear resistance characteristics near the zero point - not just in one quadrant, but in two quadrants.

As for whether MOS tubes can be used, theoretically, they can, but do MOS tubes have good enough linear resistance characteristics? I have not studied it, so I dare not say.

For information on how to obtain high linear resistance characteristics using JFET, please refer to Section 5.6 of "Transistor Circuit Design and Fabrication" by Toru Kuroda.

gmchen

By the way, I remember that the original poster asked for an oscillator with a frequency of 5M or 8M in another thread. It is not impossible to use a Wien bridge, but the frequency is a little higher. In addition, the frequency stability of the RC oscillator is not good.

S3S4S5S6

gmchen posted on 2020-7-21 21:44 By the way, I remember that the original poster asked for an oscillator with a frequency of 5M or 8M in another post. It is not impossible to use a Wien bridge, but the frequency...

The main goal is to obtain a 8~12M sine wave with the harmonic component as small as possible, which requires the sine wave to have a small distortion. If the distortion is large and the harmonic component is large, it will cause the problem of radiation emission. The problem has always been how to obtain a high-purity sine wave.

gmchen

S3S4S5S6 published on 2020-7-22 11:14 The main purpose is to get a 8~12M sine wave, and the requirement is that the harmonic component is as small as possible, which requires the sine waveform to have a small distortion. If the distortion is large, the harmonic...

Speaking of "radiated emission" here, a question comes to mind: if a certain power output is to be achieved in the end, even if the output waveform of the previous oscillator is very pure, it will still be distorted after passing through the high-frequency amplifier, that is, high-order harmonics will be generated.

Not sure how much power output you need?

gmchen

S3S4S5S6 Published on 2020-7-22 11:14 The main purpose is to get a 8~12M sine wave, and the requirement is that the harmonic component is as small as possible, which requires the sine waveform to have a small distortion. If the distortion is large, the harmonics...

In addition, for 8~12MHz, is it a single frequency or can the frequency be changed arbitrarily within the frequency band? These two situations may also affect the circuit structure.

S3S4S5S6

gmchen posted on 2020-7-22 14:34 S3S4S5S6 posted on 2020-7-22 11:14 The main purpose is to get a 8~12M sine wave, and the requirement is that the harmonic component is as small as possible, which requires the sine...

The output power is not large because the load is a capacitive load with a maximum of 15pF. The load is just a hollow metal tube with a length of 10~30cm and a diameter of 1mm.

S3S4S5S6

gmchen posted on 2020-7-22 14:36 In addition, for 8~12MHz, is it a single frequency, or can the frequency be changed arbitrarily within the frequency band? These two situations may also affect the circuit structure. ...

A frequency point in the range of 8~12M, fixed, no need to adjust

gmchen

S3S4S5S6 posted on 2020-7-22 15:27 The output power is not large because the load is a capacitive load with a maximum of 15pF, but the load is a hollow metal tube with a length of 10~30cm and a diameter of 1mm

Antenna? Electrode?