Semaphor light t5 change voltage5/10/2023 ![]() ![]() ![]() As a result, transistor T3 will conduct more current and its collector voltage will drop. This will cause T2 to conduct less current, making the DC voltage at its collector increase. When voltage at the emitters of transistors T4 and T5 rises, say, due to a temperature change, the voltage at the emitter of transistor T2 will also increase by way of R13. If capacitor C8 is not included, the feedback will be for both DC and AC voltages. To stabilise this and other voltages in the circuit, resistor R13 gives negative feedback from the output to the emitter of transistor T2. Preset VR2 is used to adjust this to half the supply voltage. The most important voltage is at the emitters of T4 and T5. This means that DC voltages for T3, T4 and T5 are all determined by the collector voltage at T2. Preset VR2 is used to adjust amplification and avoid direct coupling between transistor stages T2 through T5. The next stage is common-emitter amplifier comprising transistor T2. The FET stage in the amplifier section is used to give a high input impedance. This is necessary because human hearing follows a logarithmic response, in which a change in the output power by a factor of 10 is heard as a change by a factor of two. Like all audio volume controls, VR1 needs to have a logarithmic taper (usually denoted as ‘type C’) to give an apparent linear relationship between rotation of the control and the volume level. Since the gate terminal of FET (T1) can be regarded as an open circuit, the input impedance of the circuit is equal to the value of VR1. The value of VR1 is specified as 1-megaohm. The input signal is coupled to volume control VR1 via capacitor C1. The driver section is built around transistor BC639 (T3) while the output section is built around transistors BD139 and BD140 (T4 and T5). The amplifier section is built around JFET VHF/UHF amplifier 2N5484 (T1) and NPN transistor BC548 (T2). 2 shows the amplifier, driver and output sections.
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