Differential amplifier oscillator

A differential amplifier oscillator is an oscillator circuit for generating a sinusoidal oscillation with the feature that the oscillator works with a resonant circuit without a tap and requires few passive components. It is therefore often implemented as an integrated circuit and then only requires the actual resonant circuit in the form of external discrete components.

The circuit is also known as a cathode follower oscillator (CFO) or an emitter-coupled oscillator .

functionality

Static characteristics I (U) and G (U) of the active part

The oscillator consists of a differential amplifier with positive feedback and the resonant circuit connected in parallel . In the adjacent circuit, the resistor R1 takes over the function of the current source feeding the differential amplifier. The output power is tapped from the resonant circuit.

Due to the feedback and the limitation by the base-collector diodes, the active part has an N-shaped characteristic curve (dynatron type), with a negative differential resistance in the area of the voltage zero crossing. The oscillator oscillates when the load resistance including oscillation loss is greater than the amount of this differential resistance, for bipolar transistors the following applies:

{\ displaystyle r _ {\ text {diff}} = {\ frac {-4n * U _ {\ text {T}}} {I _ {\ text {e}}}}}

With:

Emission coefficient

{\ displaystyle n \ approx 1 \ dots 2}

Temperature stress at room temperature ${\ displaystyle U _ {\ text {T}} = {\ frac {k \ cdot T} {q}} \ approx {25 \; \ mathrm {mV}}}$
Operating current through R1 ${\ displaystyle I _ {\ text {e}}}$
absolute temperature ${\ displaystyle T}$
Boltzmann constant ${\ displaystyle k = {1 {,} 381} \ cdot 10 ^ {- 23} \, \ mathrm {Ws / K}}$
Elemental charge ${\ displaystyle q = 1 {,} 602 \ cdot 10 ^ {- 19} \; \ mathrm {As}}$

variants

Differential amplifier oscillator with FETs

The oscillator also oscillates with an inductance instead of the resonant circuit, the output signal is then approximately square-wave and the frequency depends on the operating current of the differential amplifier. ${\ displaystyle I _ {\ text {e}}}$

The circuit can be expanded to increase the collector-emitter or drain-source voltage of the transistors.

The oscillator opposite is implemented with field effect transistors . The resonant circuit consists of C1 and L1. The non-inverting input of the differential amplifier , the gate of the junction field effect transistor Q1, is connected to the resonant circuit via the grid combination C2 and R1. The inverting input, the gate of Q2, is grounded. The oscillator amplitude is determined by R2. The oscillator oscillates with the specified values at around 7 MHz.

The Lambda Diode is a similar circuit with N-JFET and P-JFET. Another component with negative differential resistance is the Gunn diode .

literature

↑ Jiří Vackář , LC Oscillators and their Frequency Stability, Tesla Technical Reports, December 1949, Chapter 4, page 6ff
↑ Günter Peltz: Two-pole oscillator circuits for parallel and series resonance, In: Funkschau, 1971, issue 15, pp. 465–466
↑ U. Tietze, Ch.Schenk: Semiconductor circuit technology . 8th edition. Springer-Verlag, Berlin 1986, ISBN 3-540-16720-X , Chapter 15.1.5.
^ NN: MECL Integrated Circuits . Motorola, Phoenix (Arizona) 1987, MC1648.
^ NN: Integrated Circuits 1976 . AEG-Telefunken, Berlin 1976, TBA570, AM / FM receiver circuit.

[1] Jiří Vackář , LC Oscillators and their Frequency Stability, Tesla Technical Reports, December 1949, Chapter 4, page 6ff

[2] Günter Peltz: Two-pole oscillator circuits for parallel and series resonance, In: Funkschau, 1971, issue 15, pp. 465–466

[3] U. Tietze, Ch.Schenk: Semiconductor circuit technology . 8th edition. Springer-Verlag, Berlin 1986, ISBN 3-540-16720-X , Chapter 15.1.5.

[4] NN: MECL Integrated Circuits . Motorola, Phoenix (Arizona) 1987, MC1648.

[5] NN: Integrated Circuits 1976 . AEG-Telefunken, Berlin 1976, TBA570, AM / FM receiver circuit.