The oscillator with electronic tuning of frequency

Radio 1991, 12

A schematic diagram of the oscillator is shown in Fig. 1. The common-base transistor stage (Q1) provides amplification of the signal in the positive feedback loop. The signal of the positive feedback develops across the resistor R1, this resistor is common to both the emitters of transistors Q1, Q2. The transistor Q2 configured as common collector circuit.

Circuit diagram of the voltage controlled oscillator with two bipolar transistors

Fig. 1
Q1, Q2 - KT503D (si, hFE = 40..120, ft = 5MHz)

The frequency of oscillation is determined by the input and output conductance of transistors and the inductance of the coil L1. Both the frequency and amplitude of the oscillation depends on the value of the resistor R1. The frequency of oscillation changes in a wide range due to changes in the conductivity of the transistors when the supply voltage changes. With the values of components, shown in fig. 1, the frequency of oscillator decreases from 700 kHz at Vcc = 3 V to 420 kHz at Vcc = 9 V (Fig. 2). To connect a low resistance load to the output of the oscillator it takes to use an emitter follower.

Voltage vs. Frequency

Fig. 2

The stability of the oscillator is relatively low, but in some cases it is not important.

S. Kurtasov

Download LTSpice model of this circuit. The Splice model of KT503D transistor:

.model KT503d NPN(Is=6.843f Xti=3 Eg=1.11 Vaf=129 Bf=106.6 Ise=66.48f Ne=1.384 Ikf=.8419 Nk=.6328 Xtb=1.5 Br=1.2 Isc=26.4p Nc=2.088 Ikr=1.637 Rb=6 Rc=1.538 Cjc=23.66p Mjc=.33 Vjc=.75 Fc=.5 Cje=30.84p Mje=.33 Vje=.75 Tr=648.9n Tf=12.74n Itf=1 Xtf=2 Vtf=30)

Copy this data and insert it in the file c:\LTspiceIV\lib\cmp\standard.bjt, then restart LTSpice if it's running.