Armstrong oscillator

A Meissner oscillator, or even Armstrong oscillator is a feedback amplifier with a frequency-determining resonant circuit, which belongs to the group of sine oscillators. The circuit is named after its inventor, Alexander Meissner, who in 1913 patented.

The Meissner Oscillator, the resonant circuit at the output of the amplifier component. In the Audion circuit by Edwin Howard Armstrong, the resonant circuit is located at the entrance of the amplifier component.

Construction

The oscillator can be implemented with various active devices as an amplifier, such as a bipolar transistor, field effect transistor, or by means of the electron tube. The actual resonance circuit is formed of a coil L2 and a capacitor C2. In addition, the voltage at the resonant circuit is a second winding L1 is magnetically coupled as a transformer L2 is returned with a suitable phase. In the first circuit, the bipolar transistor Q is used as an amplifier in the second circuit of the junction field effect transistor ( JFET) Q1.

Meissner oscillator with bipolar

Thus, the circuit generates an undamped oscillation, the loop gain must be equal to 1, and the feedback in the phase to be (0 °, or other multiples of 360 °). Since the Meissner circuit is a common emitter circuit in the circuit example of the signal is inverted by the transistor. This is undone by the transformer, since L1 and L2 have an opposite sense of winding, identified in the circuit by the two black dots, indicating how the winding start.

The first circuit shown is a common emitter circuit with current feedback, in which the gain is equal to the ratio of the collector resistor ( AC resistance of the tank circuit ) and the emitter resistor R3. Since common LC parallel resonant circuits outside the resonance frequency have very low resistance, the loop gain is greater than one for the resonance frequency. The transmission ratio of the transformer is selected so that the loop gain for the resonant resistance of the LC circuit is certainly greater than one, and the voltage at input does not override the transistor.

Meissner oscillator with JFET

In the JFET circuit consists of the resonant circuit of C2 and L2 of the transformer TR1. The JFET Q1 source circuit having a phase shift of 180 °. As with the bipolar circuit, the transformer produces an additional 180 ° phase shift. Amplitude control of the oscillator is effected by the resistor R1. R1 is connected to the amplifier input, the gate -source path and to the amplifier output, the drain -source path is connected. Because of the 180 ° phase shift between the input and output of which acts as a negative feedback R1. If R1 and selected the ratio of the transformer to match the JFET component properties, then the negative feedback ensures a stable operating point. Increasing the peak voltage across the resonant circuit, also increases the peak voltage across R1 and counteracts a further increase of the peak voltage across the resonant circuit.

The output frequency is calculated using the thomson between vibration equation:

Dimensioning

Meissner oscillators can be easily wrong size; they then swing, but the vibration differs markedly from the sinusoidal shape from. Ideally, the overall gain of the oscillator at power-on is larger than 1 and is reduced in operation at exactly 1. For this amplitude control is used when the FET characteristic that the voltage gain from the gate voltage is dependent. Other oscillator circuits, especially with more than one transistor, can be better regulated with respect to their amplitude. As in all the LC circuits, the ratio of L to be noted C, so that the circuit resonates.

Example of calculation

A typical small signal transistor is in the considered area, a DC current gain B of approximately B = 100, and a base -emitter voltage UBE = 0.65 V.

The voltage drop across R3 is 1V, so:

Thus, the voltage divider R1 and R2 must provide these 1 V plus the base-emitter voltage of approximately 650 mV. The voltage divider is = 2 mA / 100 = charged by the base current IB = IC / B 20 uA; one takes ten times the cross- current of 0.2 mA, then one can neglect the base current and receive:

For a coil with the inductance of L1 = 22 mH and a capacitor C2 = 33 nF results in an oscillation frequency of:

In order not to overload the transistor Q and produce a good sine signal, the feedback voltage must not be substantially greater than 1.5 Vpp ( voltage peak-peak). The voltage across the resonant circuit is approximately 28 Vpp into resonance. This results in a reduction of 1:18 results in a circuit for the amplification of V = 18 for which the collector resistance must be at least 9 kOhm (including the transistor output resistance of about 100 k ).

Taking as a quality factor G = 50, then the resistance of the LC circuit is at resonance frequency

This seems to be sufficient and corresponds to a resistive coil resistance of 16 Ω.

The coupling capacitors C1 and C3 have to pass only the AC voltage, and does not change the operating point of the transistor. C1 operates on the input impedance of the emitter circuit (approx. R2). C3 and the subsequent stage input resistance strain or tune the resonant circuit. A decoupling on R3 but rarely provides a good sine wave.

Application Examples

The Meissner circuit rarely finds application, since the transformer is a considerable expense; The Hartley and Colpitts circuit in some contexts, the Clapp circuit are usually preferred, especially when only one transistor is to be used. With several other transistors oscillator circuits are possible.