The Transistor in Common Emitter Configuration
In common emitter configuration, the transistor is connected such that the emitter is common to both the input and output circuits. This configuration is commonly used for amplification purposes.

The Transistor in Common Base Configuration
In common base configuration, the transistor is connected such that the base is common to both the input and output circuits. This configuration is commonly used for impedance matching and high-frequency applications.

Reverse Leakage Current in Common Emitter Configuration
Given that the reverse leakage current in common emitter configuration is 21 microamperes (μA), we can denote it as ICER. This current flows from the collector to the emitter when the transistor is reverse biased.

Reverse Leakage Current in Common Base Configuration
When the same transistor is connected in the common base configuration, the reverse leakage current reduces to UA. This current flows from the collector to the base when the transistor is reverse biased.

Calculating Beta (DC)
Beta (β) is the ratio of the collector current (IC) to the base current (IB) in a transistor. In common emitter configuration, the emitter current (IE) is the sum of the collector current and the base current (IE = IC + IB). Since the reverse leakage current is negligible in the base-emitter junction, we can assume that the emitter current is approximately equal to the collector current (IE ≈ IC). Therefore, we can calculate beta (DC) as follows:

β (DC) = IC / IB ≈ IE / IB ≈ IC / IB

Since the reverse leakage current in the common base configuration is UA, which is negligible, the base current can be assumed to be zero (IB ≈ 0). Therefore, the beta (DC) value in this configuration would be infinite.

Calculating Alpha (DC)
Alpha (α) is the ratio of the collector current (IC) to the emitter current (IE) in a transistor. In common emitter configuration, we can calculate alpha (DC) as follows:

α (DC) = IC / IE

Since the reverse leakage current in the common emitter configuration is 21 μA, we can substitute the values into the equation:

α (DC) = IC / IE = 21 μA / 21 μA = 1

In common base configuration, the emitter current can be assumed to be approximately equal to the collector current (IE ≈ IC). Therefore, the alpha (DC) value in this configuration would also be approximately 1.

Summary
- In common emitter configuration, the reverse leakage current is 21 μA (ICER).
- In common base configuration, the reverse leakage current reduces to UA.
- Beta (DC) in common emitter configuration can be calculated as IC / IB, assuming the emitter current is approximately equal to the collector current.
- Beta (DC) in common base configuration is infinite since the base current can be assumed to be zero.
- Alpha (DC) in both configurations is approximately 1 since the emitter current is approximately equal to the collector current in both cases.