Barrier RF generators on bipolar transistors. Scheme, description

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Barrier RF generators on bipolar transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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The barrier mode of operation of the transistor provides the important property that a wide variation in the values of L and C in such generators does not lead to a noticeable change in the level of the output RF voltage (0,5-0,6 V for silicon and 0,2-0,3 V for germanium ).

At first glance, the advantage of generating an RF voltage of less than 1 V is not so significant, however, this increases frequency stability (both short-term and long-term). In addition, it becomes possible to use varicals for tuning, which, at low RF voltages, degrade the stability of the oscillator frequency to a much lesser extent.

In [1], in essence, a barrier circuit of a differential amplifier is given, and in [2] a brief definition of the barrier mode of operation of a transistor is given without a detailed analysis. In this regard, let us consider some important features of the barrier mode of operation of a bipolar transistor, in which the base of the transistor is short-circuited by direct current or through a resistor with a small resistance to the collector (Fig. 1). Power is supplied to the circuit through a resistor that sets the current through the transistor, i.e. there is no usual bias circuit.

Barrier RF generators on bipolar transistors
Ris.1

A barrier transistor is a kind of diode connected in series with a current-setting resistor. Since the emitter-base voltage for a forward-biased pn junction is approximately 0,6. .0,7V for silicon transistors and 0,3 ... 0,4 V for germanium transistors, then the collector potential is equal to this value. At a saturation voltage of about 0,1V, the maximum amplitude of the output RF voltage for circuits with silicon transistors will be about 0,5 ... 0,6 V and about 0,2 ... 0,3 V with germanium ones.

The current flowing through the transistor can be approximately estimated by the formula I \u0,6d (Upit-(0,7 ... 1 V)) / R, (A), where Upit is the supply voltage, V; R is the resistance of the current-setting resistor, Ohm. In the generator circuit in Fig. 2, it is possible to remove the RF voltage from the other end of the coil. However, this circuit has a significant drawback: the LC circuit is not connected to ground at any of its ends, which makes frequency tuning with a variable capacitor almost impossible. The author proposed a circuit with a grounded capacitor (Fig. 111). Generation will also occur if C is connected between the "ground" and the base (the "base-emitter" junction is open and has very little resistance). The author successfully used such a scheme as a master oscillator for a simple FM radio microphone. Modulation was carried out using a KVSXNUMX varicap matrix.

Barrier RF generators on bipolar transistors
Ris.2

However, to generate a frequency with increased stability, it is desirable to ground one of the ends of L, which is implemented by the author in the circuit in Fig. 3, where the RF voltage can also be removed from L.

Barrier RF generators on bipolar transistors
Ris.3

Note that a change in the supply voltage (if it is not less than 1 V) at the same value of R still affects the frequency of the generated oscillations. For reliable operation of the transistor at higher frequencies, it is necessary to increase the current flowing through it by reducing V. When using KT315A, KT361A at Upit = 12 V and R = 2200 Ohm, stable operation of all the above circuits was observed at least up to 110 MHz. These circuits have high-resistance outputs and need a high-quality buffer stage and (or) removal of the RF voltage from 1/8 ... 1/10 of the turns L (counting from the grounded end), otherwise frequency instability is inevitable when the load resistance changes. The reactance Cbl at the operating frequency should be no more than 1 ohm.

Literature

1. Titze U., Shenk K. Semiconductor circuitry. - M.: - Mir; 1982, p.297
2. Stasenko V. Barrier mode of operation of the transistor.- Radio amateur 1996, No. 1, p. 15-17.

Author: Vladislav Artemenko, UT5UDJ, Kiev; Publication: N. Bolshakov, rf.atnn.ru

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