8/25/2023 0 Comments Transistor gate tablescurrent gain, ß ß = (AMI B VCE The common emitter configuration has high input impedance, low output impedance and higher current gain when compared with common base configuration. (mA) 1B WA) The current gain is defined as the ratio of a small change in the collector current to the corresponding change in the base current at a constant VCE. The transfer characteristic curve is shown in Fig. The base current 1B is increased in suitable steps and the collector current lc is noted down for each value of 1B. The transfer characteristic curve is drawn between lc and 1B, when VCE is kept constant at a particular value. Normally its value is found between 50kQ to 100kQ Its value can be found out from the input characteristic curve. output impedance, ro The output impedance of a transistor in CE mode is low. The output impedance ro is defined as the ratio of variation in the collector emitter voltage to the corresponding variation in the collector current at a constant base current in the active region of the transistor characteristic curves. In this region, E-B junction is forward biased and C-B junction is reverse biased. In the active region, the curves are uniform. Active region : The central region of the curves is called active region. In this region both base- emitter region and base-collector region are reverse biased. Below the cut off region, the transistor does not function. In the output characteristics, the region below the curve for 1B = O is called cut off region. There is very small collector current in the transistor, even when the base current is zero (1B = 0). In this region both base-emitter region and base-collector region are forward bias. (Knee point is the point, where lc is about to become a constant). (i.e) The region in between the origin and knee point. The initial part of the curve (ohmic region, OA) is called saturation region. sa turation region active region (mA) o Aver cut off on (volts) 13=60 LA 13=40 ILA IF20YA The three regions of the characteristics can be discussed as follows : Saturation region. The output characteristics thus obtained are Now, lc versus VCE curves are drawn for different values of 1B. The procedure is repeated for different values of 1B. VCE is increased in suitable equal steps and the corresponding collector current is noted. The base current 1B is kept at a constant value, by adjusting the base emitter voltage VBE. Output characteristics Output characteristic curves are drawn between lc and VCE, when 1B is kept constant at a particular value. Input impedance, ri The input impedance of the transistor in CE mode is very high. AVBF (volt) The input impedance of the transistor is defined as the ratio of small change in base - emitter voltage to the corresponding change in base current at a given VCE. The input characteristic thus obtained is shown in Fig CE Alb. 1B values are plotted against VBE for constant VCE. The procedure is repeated for different values of VCE. VBE is increased in suitable equal steps and corresponding base current is noted. C 7 7 Input characteristics cc Input characteristic curve is drawn between the base current (1B) and voltage between base and emitter (VBE), when the voltage between collector and emitter (VCE) is kept constant at a particular value.
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