At connection to an entrance of the amplifier of a source of the strengthened signal for good coordination it is necessary to know the size of entrance resistance of the amplifier. Otherwise you can receive a blockage of amplitude-frequency characteristic of a signal, strong reduction of its amplitude and emergence of various type of nonlinear distortions. It is possible to measure entrance resistance in several ways. Often at the same time measurement of size of output resistance is required.
It is required to you
- - generator of standard signals;
- - multimeter;
- - connecting wires;
- - load variable resistor 100 kOhm.
Instruction
1. The first you sposobvklyuchit a multimeter in the provision of measurement of alternating currents. Turn on the generator of standard signals in the mode in which at its exit there will be sinusoidal tension scope less than 250 mV with a frequency of 50-900 Hz. Connect the generator to an amplifier entrance. In a rupture of one of wires (consistently) connect a multimeter. The polarity of connection does not matter.
2. Increase tension of alternating current at the output of the generator up to 250 mV. If the generator has no own voltmeter, use one more multimeter. Include it for measurement of alternating voltage in the range of 2 Volta and connect in parallel to the output of the device.
3. Consider the indication of the multimeter included for measurement of alternating current. If instrument reading zero, consistently switch measurement ranges on it in the direction from bigger to smaller current. If the device shows figure 1 on the display, on the contrary, switch it to the bigger range of measurement of currents. On a formula of Ohm (R=U/I) calculate entrance resistance.
4. The generator also establish to the second sposobvklyuchita at its exit a sinusoidal signal with a frequency of 50-900 Hz in a big way of tension, equal 250 mV. Connect the generator to an amplifier entrance. In parallel connect the variable resistor (the resistor set to such position, when its resistance as much as possible) and the multimeter included in the mode of measurement of alternating voltage in the range of 2 Volta to an entrance.
5. Smoothly reduce resistor resistance until tension on an entrance of the amplifier does not fall up to 125 mV. Switch off the generator. Disconnect the variable resistor from the scheme. Include a multimeter in the provision of measurement of resistance. Measure the size of resistance of the variable resistor. This size will also be equal to the size of entrance resistance of the measured device.