Ultra-wideband amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In amateur radio practice, when building tunable voltage generators, an ultra-wideband amplifier (UWB) with electronic gain control can be useful for testing and tuning transmitters. On fig. 1 shows a diagram of such an amplifier.

Rice. 1. SSHU schematic diagram (click to enlarge)

Main characteristics

The amplifier contains an L-shaped attenuator on pin diodes VD1 and VD2, 6 amplification stages on transistors VT1, VT2, VT4, VT6, VT8, VT10 and an output voltage level sensor on the VD3 diode.

The attenuator changes the signal level at the input of the amplifying stages, thereby adjusting the gain. When the slider of the resistor R1 is moved to the leftmost position (according to the diagram), the voltage across the diode VD1 becomes zero, and its resistance is maximum. At this time, the voltage across the VD2 diode is maximum, and its resistance is minimum. In this case, the gain of the amplifier is maximum. When moving the slider R1 to the right, the gain gradually decreases. In the process of gain adjustment, a slight change in the slope of the amplifier frequency response occurs, however, in the adjustment range (34 dB), its unevenness does not exceed ±1,5 dB.

The first amplifying stage on transistor VT1 uses parallel voltage feedback through resistor R2, which reduces the input impedance of the stage and thereby increases the depth of electronic gain control. In the second stage, a first-order corrective circuit C1-R15 is used. The required mode of operation of the first two stages (quiescent current - 5 mA) is set by selecting resistors R3 and R4.

In the remaining stages on transistors VT4, VT6, VT8, VT10, active collector thermal stabilization is used [1]. The quiescent currents of these transistors are selected equal to 50 mA (for VT4 and VT6), 400 mA (for VT8 and VT10), and they are set by selecting resistors R5 ... R8. The required bandwidth in the cascades on transistors VT4, VT6 and VT8 is achieved through the use of third-order reactive interstage corrective circuits [2].

The output stage is made according to the scheme with voltage addition [3] and provides summation at the load of signal voltages given by transistors VT8 and VT10.

The printed circuit board of the amplifier (Fig. 2) consists of two parts with dimensions of 65x45 mm and 82x45 mm. It is made of double-sided foil fiberglass with a thickness of 1,5 ... 2 mm. The dotted line in fig. 2 shows the places of metallization of the ends of the board, which can also be done using metal foil, which is soldered to the bottom and top parts of the board. Metallization is necessary to eliminate parasitic resonances and ground the desired sections of the printed circuit board. After metallizing the ends with a file, the lower part of the board is leveled, and it is installed in the case.

Rice. 2. Amplifier circuit board

Setting up the amplifier consists of several stages. First, the frequency response of the amplifier is cascaded. To do this, with the help of resistors R3 ... R8, the quiescent currents of the transistors are set. Then, as a load VT1, a 50 Ohm resistor is connected through an isolation capacitor. By selecting R2, a uniform frequency response of the cascade is achieved up to a frequency of 250 MHz. Further, the second is connected to the first stage, and by selecting the capacitance of the capacitor C1, the frequency response of the first two stages is equalized. After connecting the cascade on the transistor VT4, by selecting C2, a uniform frequency response of the first three cascades is established in the low and medium frequencies. The selection of C3 equalizes the frequency response in the high frequencies. If this cannot be achieved, the value of C2 should be reduced.

Then the next cascade is connected, and the tuning process is repeated. After connecting the cascade to VT10, using C4, the maximum output power of the amplifier is achieved. Roughly, this condition corresponds to the voltage transfer coefficient of the output stage, equal to two [3].

The photograph shown in Fig. 3 shows a panel with 5 pins, to which the power supply voltage of the amplifier is supplied, and also the potentiometer R1 and the dial indicator of the output power level PV1 (M4761) are connected. Correction of the indicator readings is carried out using R9.

Fig.3. Amplifier circuit board

Literature

1. Titov A.A. Calculation of the active collector thermal stabilization circuit and its use in amplifiers with automatic control of the consumed current. - Electronic technology, 2001, N2, p.26
2. Titov A.A. Parametric synthesis of an interstage corrective circuit of an ultra-wideband power amplifier. - Proceedings of universities. Electronics, 2002, N6, p.81.
3. Babak L.I., Dyachko A.N. Powerful nanosecond video amplifier according to the circuit with voltage addition. - Devices and technique of experiment, 1981, N3, S.127.

Author: A.Titov, Tomsk, titov_aa@rk.tusur.ru; Publication: cxem.net

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