Double-gate MOS tubes, as the name suggests, are MOS tubes with two gates. Compared with ordinary field tubes and transistors, they have many advantages. Therefore, they are very common in the FM high amplifier and mid-amp circuits of some famous classic machines. Because of this

double-gate MOS tube, as the name suggests, is a MOS tube with two gates. Compared with ordinary field tubes and transistors, it has many advantages. Therefore, it is very common in the FM high amplifier and mid-amp circuits of some classic machines. . Because of this, I also concluded from experience that any machine that uses dual-gate MOS is a good machine [smile].

double-gate MOS tube has two gates G1 and G2, which can be conveniently used for mixing or AGC control. The reason why double-gate tubes can be used for high-amplification is mainly due to several characteristics of it: 1. The input impedance of the MOS tube is high, and the antenna loop access is flexible and convenient; 2. By controlling the bias voltage of G2 The drain current can be controlled, thereby controlling the amplification gain or transconductance, without affecting the input impedance of the signal grid G1 to avoid detuning; 3. The antenna signal can be injected into G1, and the local oscillator can be injected into G2 to achieve frequency mixing; 4. The MOS transistor has low noise, has a larger dynamic range than the transistor , and has strong resistance to cross-modulation.

This article combines the high amplifier circuit of the FM head of two classic models Sony SW55 and Yajia AT-93 to analyze the specific application of dual-gate MOS tubes. Sony SW55 and Yajia AT-93 are both classic and famous machines, and Yajia AT-93 is a high-end radio head with 5-link electronic tuning. The two frequency modulation head circuits just represent two typical uses of double-grid tubes.

Sony SW55

SW55 uses Sanyo 3SK132A as the FM high amplifier tube, which is an N-channel dual-gate MOS tube dedicated to FM TV.

SW55 high-amp circuit

As can be seen from the figure, the antenna signal is coupled to through the T9 coil and then enters G1. Because the S pole is directly connected to the ground and there is no source resistance, G1 is 0 biased. The bias voltage of G2 is 1V, which is obtained by dividing the voltage by 4.7K and 2.2K resistors. The B+ power supply here is not regulated, so the sensitivity of SW55 is relatively sensitive to the battery voltage. After the voltage drops, the sensitivity, sound Performance is somewhat degraded. Since the Vg2 voltage here is fixed, the transconductance of the tube, that is, the gain, is fixed. From the technical data of the tube, it can be estimated that when Vg1=0, Vg2=1V, and Vds=3.2V, the transconductance of the tube is It is less than 5mS (because Vds10V) and the gain of the high amplifier stage is very low. The drain output is connected to the coil tap to achieve impedance matching and at the same time reduce the impact of the output resistance of the tube on the loop. The two sets of LC frequency selective tuning at the front and rear of the high-amplifier tube use varactor diodes , but back-to-back varactor diodes are not used.

3SK132A transfer admittance

Yajia AT-93

AT-93 high-amplification circuit

AT-93 is a high-end radio head. Its FM head uses 5-link tuning (for the convenience of analysis, only the high-amplification circuit is posted). Sanyo 3SK107 double-gate N-channel MOS tube is also a tube dedicated to VHF. It can be seen that the biggest difference from SW55 is that the G2 gate voltage is not fixed, but the connected AGC voltage. This voltage comes from the mid-stage amplifier or the frequency identification DC voltage . When there is no signal, the AGC voltage is high. , the drain current is large, the transconductance is also large, and the gain is also large. When the signal is received, the greater the signal strength, the lower the AGC voltage, the smaller the drain current, and the transconductance gain is reduced accordingly, achieving AGC control. Yajia uses back-to-back varactor diodes for tuning. Since the relationship between the capacitance of the varactor and the bias voltage is not completely linear, especially for AC positive and negative The response to half-cycle interference is also asymmetric, so this back-to-back usage takes advantage of the reverse complementary characteristics of the two tubes to improve the varactor's response to strong signals or interference, thus Greatly improve problems such as nonlinear distortion and loop detuning. Many high-end machines use this method, such as the transfer admittance characteristics of Grundig S700.

3A107. From the working points of the above two tubes combined with the characteristic curves, it can be seen that the offset of the FM high-amp tube of , Sony 's SW55 is too high. It's too low, so the gain is too low, which doesn't seem right.In order to verify this problem, I specifically checked and measured the actual circuit, and found that Sony did not actually produce it according to the drawings. The actual voltage of G2 is 2.5v, which is much larger than the 1v indicated on the drawing, so its transconductance gain can basically meet the requirements. I reduced the R119 resistor to 1k, further increased Vg2 to 2.6v, and found that the FM gain did increase a little bit, and the number of weak station signal grids increased by two grids. Although it is very limited, the designed drain-source voltage is only 2.7v. It can only be the.

Actual circuit of Sony SW55