DC coupling circuit b that raises the base potential of VT2

DC coupling circuit b that raises the base potential of VT2

The operating points of each stage in the DC-coupled amplifier circuit are not independent and influence each other, resulting in zero point drift
.
    The basic function of the amplifier is to stably amplify the input signal. When there is no input signal or the input signal is zero (the input terminal is connected to ground), the output
should maintain a certain voltage value or zero value. However, due to changes in ambient temperature or supply voltage, the output of the amplifier fluctuates or changes. This
phenomenon is called zero-point drift.
    In fact, due to temperature changes, power supply voltage fluctuations and transistor aging, changes in transistor parameters are an objective reality,
so it is inevitable to cause zero-point drift in amplifier circuits at all levels. However, in a resistor-capacitor coupling circuit ( a coupling circuit
in which amplifiers are connected by capacitors rather than directly ), due to the role of the coupling capacitor, this drift is limited to the range of this stage and will not affect the next stage, let alone the next stage. Zoom in step by step. However, in a DC-coupled amplification circuit, a small change (drift) in the first stage will affect the next stage and even amplify it step by step, causing serious drift at the output end, as shown in Figure 1-42.     This circuit is a humidity detection circuit. Changes in the impedance of the humidity sensor will cause changes in the output voltage, which will become changes in voltage after amplification.     The zero-point drift of an amplifier circuit is usually measured by converting the drift voltage of the output terminal into the drift amount of the input terminal. That is, the ratio of the zero-point drift voltage of the output terminal to the voltage amplification factor of the amplifier circuit is used as the zero-point drift voltage indicator of the amplifier circuit. The smaller the value, the better the circuit quality.     The zero-point drift mainly affects the output of the first stage. Therefore, to suppress or reduce the zero-point drift of the amplifier circuit is to suppress or reduce the zero-point drift of the first stage. The measures usually taken include the following aspects.     ④Choose high-quality silicon transistors with good stability.     ② Use single-stage or inter-stage negative feedback circuit to help reduce zero drift.     ③Use reverse-changing thermal elements to compensate for the zero-point drift of the amplifier tube caused by temperature effects. ④The use of differential amplifier circuits can effectively suppress zero point drift and is the main form of DC amplifier circuits.