Cascode amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Cascode RF amplifiers are widely used in modern circuitry, since they have a number of advantages and, first of all, high resistance to self-excitation. The magazine "Rvdio" has repeatedly published descriptions of such amplifiers and devices with their use. We present to readers one more feature in the guise of a classic quad-code amplifier.

Known cascode amplifiers usually have a relatively low input impedance and are often quite difficult to set up. The introduction of an automatic gain control (AGC) into them is also not always easy.

The cascode amplifier described in [1] (Fig. 7.13) is free from these shortcomings. It is made according to the common source-common emitter scheme using a "current mirror" (Fig. 1) and a DC connection of the steps. The use of a matched pair of transistors VT2, VT3 in the "current mirror" makes it possible to bring the amplifier in terms of temperature stability almost to the level of a step on a field-effect transistor VT1, and the full use of the supply voltage significantly expands the amplitude characteristic. The linearity of the amplifier as a whole is largely dependent on the linearity of the FET and, as shown below, can be improved.

The control characteristic of the amplifier also has a number of positive features, in particular, it is more linear, which is typical for steps on field-effect transistors. Gain control in the device is easy to implement, for example, by replacing the resistor R1 with a collector-emitter section of a bipolar transistor or by closing the field-effect transistor VT1 through the gate circuit.

The input transistor VT1 provides the required input impedance and does not load the input bandpass filter L1C1. The low input impedance of the "current mirror" virtually eliminates parasitic positive feedback in the amplifier and allows you to turn on the resonant load L2C4 directly at its output. Positive factors include the fact that the input and output bandpass filters are "tied" to a common wire, which greatly simplifies the cascading of the amplifier, for example, when creating multistage intermediate frequency amplifiers of superheterodyne radio receivers on its basis.

The linearity of the amplifier as a whole, as well as the linearity of regulation, as well as "decoupling", in particular, can be significantly improved if it is assembled according to the common source-common base scheme (Fig. 2), using the simplest RF isolation transformer T1 according to [2 ]. Note that by turning on the transformer in an appropriate way, it is possible to ensure the phase inversion of the output voltage or the absence of magnetization of the magnetic circuit. On fig. 2, the transformer is turned on without magnetization.

For a comparative evaluation of the cascode amplifier options, digital (using the ELECTRONICS WORKBENCH program) and physical models of the amplifier and its prototype were tested using available radio components - transistors KP303B, KT361V and a transformer wound on a K7x4x2 ring made of ferrite with a magnetic permeability of 1500 with two windings 15 turns of wire PEV-2 0,2 [2]. The inductance of the primary winding is controlled instrumentally.

The filters of the IF amplifier of the transistor radio "Serenade-406" were used as bandpass circuits. The selection of components by parameters was not carried out. The current consumed by the amplifiers was not controlled. The operating point of the field-effect transistor was set by changing the resistance of the resistor R1 in decades within 100 Ohm...10 kOhm. Measurements were taken with an oscilloscope C1-55.

The results of the experiment are shown in fig. 3, which shows the dependence of the gain on the resistance of the resistor R1. Curve 1 corresponds to the digital model of the amplifier according to the circuit in Fig. 2; 2 - its physical model; 3 - physical model of the prototype (see Fig. 1). Amplifiers work steadily and without distortion in all dynamic range. The low gain is due to the reduced equivalent resistance of the output bandpass filter.

The common-source-common-base stage gain (see Fig. 2) is determined with good accuracy by the product of the field-effect transistor transconductance and the current transfer coefficient of the bipolar transistor, measured at the operating point, and the equivalent resistance of the band-pass filter.

In conclusion, it can be noted that the use of an amplifier according to the common source-common base scheme, which has the best parameters in terms of linearity, gain, depth of its regulation (up to closing) and manufacturability, is more preferable. Nevertheless, all amplifiers are operational, do not require the establishment and selection of transistors (tuning of band-pass filters, of course, is necessary), they are well cascaded. You can adjust the gain both in the gate circuit of the field-effect transistor (at zero power) and in the source circuit by changing the resistance of the resistor up to the closing of the amplifiers.

Literature

1. Goroshkov B. I. Elements of radio electronic devices (Handbook). - m.: Radio and communication, 1989.
2. Ed. E. T. Circuitry of radio receivers. Practical guide (Translated from German) - M .: Mir, 1989.

Author: V.Guskov, Samara

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