Stable high frequency amplifier. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Stable high frequency amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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The UHF circuit diagram of which is shown in Fig. 1 is distinguished by a large and stable gain Ku (several thousand) and low consumption and high stability of characteristics [1].

For direct current, the transistor VT1 is turned on by a diode (the base and collector are closed through the inductor L1). The whole DC circuit is an improved Wilson current generator with deep current feedback. Therefore, the operating modes of all transistors are stable and do not depend on temperature and supply voltage (if it is more than 2 V). The collector currents VT2 and VT3 are the same and are set by resistor R1, the value of which is determined by the formula: R1 \u2d (Upit-0.7Ube) / Ik, where Ube= 2 V for silicon transistors; Ik - the desired collector current VT3, VTXNUMX.

In terms of alternating current, the circuit is an OE-OE-OE amplifier, which has the highest coefficient compared to other transistor switching circuits in three-transistor amplifiers.

The first amplification stage is made on the transistor VT1, and L1 provides a bias. A feature of the operation of VT1 is the equality of the collector-base voltage of this transistor to zero. However, this does not affect its amplifying properties, since the collector-emitter voltage Uke = 0.7 V, which for low-power silicon transistors is much higher than the saturation voltage, which is 0.1-0.2 V.

Stable high frequency amplifier

C1,C2= 0.033uF, L1,L2= 40uH, R1= 5.1kOhm

With such a low voltage Uke, the active mode VT1 is maintained only as long as the amplitude of the alternating voltage on its collector does not exceed 200 mV. However, in this amplifier, it cannot be greater due to the high gain of the subsequent stages on VT2, VT3.

The second amplification stage on VT2 is also made according to the OE scheme, the amplified signal is allocated to R1. From the collector VT2, the signal goes to the base VT3, the emitter of which is grounded at high frequency through the blocking capacitor C2. The amplified VT3 output signal is allocated to the load Z1. which can be used as a parallel oscillatory circuit (with a resonant amplifier) or a resistor (with a broadband one).

The amplifier has a high and stable gain, is not critical to the installation and arrangement of elements. With the ratings indicated in Fig. 1, VT1-VT3 KT368AM, Upit \u12d 1V and a resistor of 0k1m as Z0.1, the amplifier has a frequency band of 20-3000 MHz and Ku \u1.5d 12. If it is necessary to adjust Ku over a wide range (for example, for AGC), the control voltage of +1 ... XNUMX V can be applied to the upper terminal RXNUMX by disconnecting it from the plus of the power source.

Stable high frequency amplifier

An example of using the considered UHF in a direct amplification receiver in the MW range is shown in Fig. 2. The magnetic antenna WA1 is made on a rod 400NN 0 8 mm L1 contains 90 turns of PELSHO 0.1 wire, and L2 - 3 turns of the same wire. chokes included resistors R2, R2. From the UHF input Ku=3 at Upit=6000 V. The detector is made on diodes VD6-VD1, which must be germanium. The receiver remains operational when the voltage drops to 3 V.

Literature

1. G. Utochkin. I. Goncharenko. High frequency amplifier. Application of the USSR No. 4649488 / 09, Decision of VNIIGPE on the issuance of a copyright certificate dated 23.11.89/XNUMX/XNUMX

Authors: G. Utochkin, Ryazan, I. Goncharenko (RC2AV), Minsk; Publication: N. Bolshakov, rf.atnn.ru

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Sergei
Collected according to 1 scheme. It worked, but the noise became large, it was powered by 5 volts. I tried to put more powerful transistors - it did not help. Strange why?

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