Impedance transformer network circuit structure

Abstract

The invention discloses an impedance transformer network circuit structure. Three JFET tubes are cascaded to form a source follower, two triodes are cascaded to form a buffer, the input capacitance of the source follower and the output capacitance of the buffer can be reduced, the bandwidth is increased at the same time, the external interference is reduced, the isolation degrees between channels is improved, and the occurrence of high-frequency oscillation is inhibited.

Description

technical field [0001] The invention relates to the field of electronic circuits, in particular to an impedance transformation network circuit structure. Background technique [0002] In instruments such as oscilloscopes, the input impedance is often required to be 1MΩ or higher, but high-bandwidth amplifiers often have low input impedance. In order to meet the two requirements of high input impedance and high bandwidth at the same time, we will add a first-stage active network before the amplifier. The network has high input impedance, low output impedance, and a gain close to 1 to achieve high-impedance and low-impedance conversion. , we become the impedance transformation network. [0003] At present, the common implementation scheme is: a unity-gain amplifier composed of a source follower and an emitter follower, using a low-frequency feedback loop to adjust the bias voltage and improve the DC characteristics. The frequency of the DC feedback loop is limited to DC and l...

AI Technical Summary

Problems solved by technology

[0005] 1. Most of the protection diodes of this network use diode BAV99 at present, mainly because the junction capacitance of this diode is small, and the input capacitance of this network can be reduced by using this diode, but the leakage current of this diode is relatively large

This leakage current will cause an error voltage. In the low frequency and DC frequency bands, the final output of the overall network is regulated by the op amp. In the high frequency band, the overall network output is obtained by the first capacitor directly acting on the unity gain amplifier. However, in the high frequency and low frequency In the excessive frequency band, and the frequency band where a filter gradually begins to function (usually 50Hz ~ 1KHz), the error voltage will cause the gain of the final output signal to change, and because the leakage current will change with the temperature, resulting in this The overall gain of the band varies with temperature

There is also a plan to replace the diode with BAV199. The diode has a small leakage current, but its junction capacitance is large, which will increase the input capacitance of the entire network, thereby affecting its bandwidth.

[0006] 2. The source follower is not ideal. According to the performance of the JFET tube, a relatively simple current source will cause the current of the JFET tube to be affected by V DS effect, and as the DC bias of the input signal varies, V DS There must also be changes, which will cause the quiescent operating point of the current source to change as the DC bias voltage of the input signal changes, causing the output signal gain to be affected by the input signal bias voltage

At this time, the input signal makes the V of the first capacitor DS less than the V of the second capacitor DS , it will cause the first capacitor current to over-current, resulting in V GS forward bias, causing damage to the first capacitor

[0007] 3. The second-stage buffer uses a simple emitter follower, and the DC bias of its input signal will affect the static operating point of the buffer, thus affecting the gain and linearity

[0008] 4. The output load capacitance value will affect the overall bandwidth of the buffer network at high frequencies

[0009] 5. In applications similar to oscilloscopes, in most cases there are multiple analog channels, and multiple such circuits need to be used together. In the case of high frequencies, the power supply becomes a channel for each channel to interfere with each other, resulting in Reduced channel-to-channel isolation

[0010] 6. In the case of radio frequency, the power supply can no longer be regarded as grounding, and the phase influence of the PCB microstrip line on the signal must be considered

In order to have better bandwidth performance, the second-level buffer network of the network will use radio frequency triodes. The bandwidth of these triodes will reach more than 8G. The delay of the wiring will form a positive feedback to the signal of a certain frequency, causing the network to oscillate.

If the triode is selected with a lower frequency triode, it will have an impact on the overall bandwidth

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