Are there any methods and techniques for testing analog circuits?

The characteristic of the switch circuit is that it works in cut-off-amplification, amplification-saturation cut-off-saturation and other states. Their DC voltage and DC current vary with the presence or absence of a signal, that is, the DC voltage and DC current under signal and no signal are different. These circuits are non-linear, and a DC component will be generated when the signal passes. It is this new DC component that changes the original working state and enters a new working state. The static and dynamic voltage difference we measured also reflects this fact. According to this voltage change, it is estimated whether the signal enters the circuit under test.
Due to the low sensitivity and low internal resistance of the AC voltage file of the multimeter, the AC file can only be used to test the output voltage of certain parts with a certain output capability. The sinusoidal AC voltage can be tested directly with the AC file of the multimeter. Generally, the voltage in the circuit often contains pulsating components, that is, AC and DC exist at the same time. In this case, the DC must be cut off to test the AC voltage. The specific method is the AC of the multimeter. A capacitor is connected in series with the gear.
For some circuits, sometimes it is necessary to test the current of the circuit to determine whether the circuit is working properly. When testing DC current, you can either disconnect the circuit and connect it to the ammeter, or you can calculate the current value by measuring the value of the relevant voltage and resistance in the circuit without cutting off the circuit.
Are there any methods and techniques for testing analog circuits?
We know the proper working state of the circuit. In order to identify whether the circuit is in this state, it is necessary to master the method of judging the working state based on the voltage value. The working state of the transistor is determined by the bias direction of the emitter junction and the collector junction, and the bias direction can be judged by measuring the voltage of the three pins. It should be noted that the above judgment method is not suitable for the circuit of oscillation circuit and BE junction reverse bias. Because their reverse bias voltage is generated by the signal itself through the BE junction, not external.
The bias state of the oscillator transmit junction is generally insufficient forward bias and reverse bias state. If the BE junction voltage reaches the normal bias voltage, the oscillation stops.
Because there is a certain resistance between the power supply and the collector, when the collector current changes, the collector voltage will change accordingly. Therefore, after changing the bias state, observing the change of the collector voltage can deduce the change of its current, and further judge whether the transistor has the ability to amplify. Regardless of whether there is a signal or not, the operating point of this type of amplifying circuit will not change, so this method is feasible. The method is to short-circuit the BE junction of the tube under test, and it should appear:

UBE=0V,IB=0,IC=0. UCE=VCC. UE0.
means that the transistor is like an open circuit. But this method cannot be used for direct coupling circuits, because this method will cause the circuit to work abnormally. For this kind of circuit, when there is no signal, it is saturated. This is also possible to observe the change of UC by short-circuiting BE junction. The transistor working in this state is already off when there is no signal. Only an external bias current can change the state. The method is to connect a resistor between the base of the tube under test and the power supply or with the ground to get a positive bias. The resistance can be between 5 and 10 kilohms.