Engineering Dictionary
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Transistor Buffer

A Buffer Amplifier is an amplifier that isolates one circuit from another, and could provide amplification. A buffer might be used to impedance match an input circuit to an output circuit. In the circuit example a transistor buffer is used to decrease the loading effect on an oscillator [the input circuit which is not shown] by reducing the interaction between the load and the oscillator.

This buffer uses an NPN transistor as the amplifier. The selected transistor needs to operate at the selected voltage supply and be able to handle the current required by the load. No example transistor part number is provided because the component selection would depend on the circuit parameters, which are unknown in this case.

NPN Transistor Buffer Amplifier

Buffer Circuit Description

The buffer amplifier is made by an NPN Transistor in a Common-Emitter configuration. Basically any transistor will work for this type of circuit, a 2N2222 transistor is a common NPN transistor which could be used. The input which is an oscillator in this example sees a high impedance load via C1 and R2. The resistor R2 would need to be a high value resistor, at least 10 times the values of the output impedance of the circuit being connected to the buffer. Because the transistor inputs appears as a high-impedance it does not load-down or effect the circuit connected to it. However the output load sees R3 as the impedance, which is a lower resistance. The reverse logic applies here, resistor R3 should be a low resistance, having a value of 10 times lower than the input impedance of the circuit being connected. The input and output are in phase because the output is being derived from the emitter side of the output. The resistor R1 is a bias resistor, to bias the base of the transistor.

Transistor Design

Many of the circuits on this site don't show Vcc power supply filter capacitors, also called By-pass capacitors or Decoupling capacitors. Good design practice dictates that every semiconductor or transistor have at least one by-pass capacitor from Vcc to ground. Normally a 0.1uF capacitor should work well in most cases, but the value depends on how noisy the Vcc line is and how much current the component requires. When possible always use short wires between the filtering components and the semiconductor. Longer wires between the decoupling capacitor and the transistor or IC just add more inductance between the devices and reduces the effect and response of the capacitor.

NPN Transistors

This site provides details for many different NPN Transistors, however only transistors, in combination with their packages, that comply with military temperature ranges are listed. So many common commercial transistors that are only offered in plastic packages are not listed. However data on all the different Transistor Packages are provided, showing package dimensions, case out lines and their common uses.

PC motherboard

Distributor rolodex Electronic Components Electronic Equipment EDA CDROM Software Engineering Standards, BOB card Cabled Computer Bus Electronic Engineering Design Table Conversion DB9-to-DB25.
Distributors Components Equipment Software Standards Buses Design Reference