{"id":2357,"date":"2024-07-29T14:13:14","date_gmt":"2024-07-29T08:43:14","guid":{"rendered":"https:\/\/study.madeeasy.in\/?p=2357"},"modified":"2025-09-08T14:58:37","modified_gmt":"2025-09-08T09:28:37","slug":"small-signal-model","status":"publish","type":"post","link":"https:\/\/www.madeeasy.in\/study\/ec\/analog-circuits\/small-signal-model","title":{"rendered":"Small Signal Model of BJT"},"content":{"rendered":"

It is the equivalent circuit of BJT and it is used in analysis of small signal model of BJT amplifiers.<\/p>\n

\"Small<\/p>\n

Transistor Hybrid Model<\/h2>\n

The basic assumption in arriving at transistor linear model or equivalent circuit is that the variations about quiescent point are assumed small, so that the transistor parameters can be considered constant over signal excursion. The operating (quiescent) values of current and voltage are determined by method employed to bias the transistor. These values do not enter into an incremental model, which is used only to find small-signal variations about the Q-point.<\/p>\n

The transistor model presented here is given in terms of h-parameters, which are real numbers at audio frequencies, are easy to measure, can also be obtained from the transistor static characteristic curves, and are particularly convenient to use in circuit analysis and design. Furthermore, a set of h-parameters is specified for many transistors by the manufacturers.<\/p>\n

The h-parameter model of BJT is defined by following equations for the circuit shown in figure below.<\/p>\n

\"Transistor<\/p>\n

Typical h-Parameters Values for a Transistor (at IE ~1.3 mA)<\/p>\n\n\n\n\n\n\n\n\n
Parameter<\/th>\nCE<\/th>\nCC<\/th>\nCB<\/th>\n<\/tr>\n
\n

h11<\/sub><\/em>\u00a0= hi<\/sub><\/em><\/p>\n<\/td>\n

1,100 \u2126<\/td>\n1,100 \u2126<\/td>\n21.6 \u2126<\/td>\n<\/tr>\n
\n

h12<\/sub><\/em>\u00a0= hr<\/sub><\/em><\/p>\n<\/td>\n

2.5 \u00d7 10\u20134<\/sup><\/td>\n~1<\/td>\n2.9 \u00d7 10\u20134<\/sup><\/td>\n<\/tr>\n
\n

h21<\/sub><\/em>\u00a0= hf<\/sub><\/em><\/p>\n<\/td>\n

50<\/td>\n\u201351<\/td>\n\n

\u20130.98<\/p>\n<\/td>\n<\/tr>\n

h22<\/sub><\/em>\u00a0= ho<\/sub><\/em><\/td>\n24 \u00b5A\/V<\/td>\n25 \u00b5A\/V<\/td>\n\n

0.49 \u00b5A\/V<\/p>\n<\/td>\n<\/tr>\n

1\/ho<\/sub><\/em><\/td>\n40 k<\/td>\n40 k<\/td>\n\n

2.04 Mho<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Approximate Conversion Formulas for Hybrid Parameters<\/p>\n

\"<\/p>\n

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