Please help me analyze this circuit.

First, the static operating point is analyzed. The base of Q1 is about 2.3V, the conduction voltage is calculated as 0.6V, the current flowing through the emitter and the collector is about 1.5mA, the static operating point of the collector is about 9V, and the Q1 magnification is 2 times. The advantage of the base circuit is that it will broaden the frequency characteristics of the amplifier circuit. Analyze the following circuit in the same way, the second-stage emitter follower, the third-stage common emitter amplifier circuit, and the fourth-stage transistor switch circuit? Input high level and output high level. When low level, output is low level.

D1 and D2 are clamp circuits, which limit the level, and are estimated to be protective.

The limiting function of D1 and D2 is also called clamping; Q2 and Q3 are resistance-capacitance coupling circuits.