AF with telegraph filter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The audio frequency amplifier, the circuit of which is shown in fig. 1 is intended for simple communication devices - superheterodynes and direct conversion receivers.

The gain of this UZCH is about 1000 (60 dB). Bandwidth from 250 to 2700 Hz (by level - 6 dB). To receive telegraph signals, it can be narrowed down to 300 Hz with an average frequency of about 900 Hz.

UZCH is made on the operational amplifier DA1, the mode of operation of which for direct current sets the divider on the resistors R1, R2. The audio frequency signal is fed to the non-inverting input of the op-amp, and a feedback signal is fed to its inverting input from the output of the op-amp. It passes through RC circuits that determine the gain of the device and its frequency response (AFC).

When the contacts of switch SA1 are open, the frequency response of the amplifier is formed by resistors R3, R4 and capacitors C2, C6. At medium frequencies (1 ... 2 kHz), the gain K is determined only by resistors R3 and R4. Since the signal is applied to the non-inverting input, then K=1+R3/R4. When shown in Fig. 1 ratings of these resistors, the gain will be about 1000. Note that 1000 is the maximum allowable gain of the ultrasonic frequency converter when using the K140UD8 operational amplifier and some other internally corrected operational amplifiers. This is illustrated in Fig. 2, which shows the frequency response of the op amp itself. It can be seen that at high values ​​of the gain, even without taking into account the influence of the add-on elements, the bandwidth will already be less than the required 3 kHz.

The frequency response of the amplifier at low frequencies is primarily formed by the R4C2 chain. At the frequency F=1/2pR4C2 the gain will decrease by 3 dB with respect to the mid frequencies. It is easy to verify that with the ratings indicated in the diagram, this will happen at a frequency of approximately 280 Hz.

At high frequencies, the frequency response of the amplifier will mainly determine the frequency response of the operational amplifier DA1 (Fig. 2).

You can additionally attenuate high frequencies by connecting a capacitor (C3) in parallel with R6, the capacitance of which is selected experimentally. If the op-amp itself did not effectively “fill up” frequencies above 3 kHz, then the capacitance of this capacitor with the value of the resistor R3 indicated on the diagram should be about 1000 pF (it is calculated using the same formula as in the previous case). Taking into account the real frequency response of a particular instance of the op-amp, in practice, the capacitance of this capacitor will be less. In particular, there may be no "double T-bridge" at all, which is formed by two T-shaped chains (R6R7C8 and R8C7C9) connected in parallel.

The dependence of the signal transmission coefficient of a double T-bridge on frequency is shown in fig. 3.

At a certain frequency (it is customary to call it the quasi-resonance frequency), the transmission coefficient of such a circuit decreases significantly - by a hundred or more times -. If a double T-bridge is connected to the feedback circuit of our amplifier in parallel with resistor R3, then at the quasi-resonance frequency the bridge will practically not affect the UHF transmission coefficient as a whole. At frequencies above and below this frequency, negative feedback will increase (double T - the bridge, as it were, shunts the resistor R3), reducing the gain of the amplifier. As a result, a "resonant" AFC is formed (curve 1 in Fig. 4). The same figure shows the frequency response of the amplifier with the double T-bridge disabled (curve 2). For the level of 0 dB in this figure, the gain of the UZCH at a frequency of 1 kHz is taken.

The quasi-resonance frequency of a double T-bridge is determined by the values ​​of its elements. When the conditions C = C7 = C8 = C9 and R = R6 = R7 = 4R8 are met, it can be calculated using the formula F = 0,45/RC. Within small limits, the quasi-resonance frequency can be changed by selecting only one resistor R8.

Resistor R5 - decoupling. It reduces the load on the bridge with a relatively low resistance resistor R4. If it is not installed, then the narrowing of the UZCH bandwidth when connecting a double T-bridge will be significantly less, i.e. filter will be ineffective. By selecting this resistor and controlling the frequency response of the amplifier, it is possible to set the bandwidth of the ultrasonic frequency when receiving telegraph signals in accordance with the individual tastes of the operator.

The use of an operational amplifier in an ultrasonic frequency converter gives one advantage - the design assembled from serviceable parts does not require adjustment. If the amplifier "did not go" from the first turn on, then you need to check the op-amp mode for direct current. The voltage at its output (pin 7) should be close to half the voltage of the power source (it is set by the divider on resistors R1 and R2). If this is not the case, then either you made mistakes during installation or selection of elements for the structure, or the op-amp is simply faulty.

When repeating the design, most modern and not so modern operational amplifiers can be used. If an op amp is used without field effect transistors at the input (for example, K140UD7), then it is advisable to reduce the resistance of resistors R1 and R2 to about 100 kOhm, while maintaining the condition R1 = R2. Oxide capacitors can be of any type.

The amplifier is designed for use with headphones with an impedance of 50...100 ohms. If the radio amateur has headphones with less resistance, then a small output stage will have to be added to this amplifier. The supply voltage of this UZCH is 9 ... 12 V.

A gain of 1000 is more than sufficient for an ultrasonic superheterodyne receiver. For a direct conversion receiver, the total gain along the audio frequency path must be a hundred times greater, therefore, the ultrasonic frequency converter, the circuit of which is shown in Fig. 1, in this case, applications must be supplemented with a pre-amplification stage. Its scheme is shown in Fig. 5. It is made on a transistor, working to reduce the level of its own noise in a mode with a small collector current (about 0,2 mA). The gain of such a stage is determined by the ratio of the load resistance in the collector circuit of the transistor VT1 (mainly R3 and R7 connected in parallel) and the sum of the resistances of the resistor in the emitter circuit, not shunted by the capacitor (R4), and the resistance of the emitter junction. The latter can be estimated by a simple formula Re = 25/I. If we substitute the current in milliamps into this formula, then the resistance will be in ohms. With an emitter current of 0,2 mA, the resistance Re will be 125 ohms. It is not difficult now to estimate the gain of this cascade - about 80.

When calculating the gain of such a cascade, one should not forget about the input impedance of the UHF cascade following it. But in our case, it can be safely neglected - it is about 200 kOhm (the resistance of resistors R1 and R2 connected in parallel is in Fig. 1). Given such an input impedance of the subsequent stage, the gain of the preamplifier will decrease slightly - to 75.

Capacitor C4 limits the bandwidth of the preliminary stage from above by a value of 4 ... 5 kHz.

For orientation in Fig. 5 shows the modes for direct current at a power supply voltage of 12 V. If it is less, then you need to take a filter resistor in the power circuit of this stage (R6) with a lower resistance.

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