lab3 - Experiment-3 Experiment-3 Multi-Transistor...

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Experiment-3 R. B. Darling EE-332 Laboratory Handbook Page E3.1 Experiment-3 Multi-Transistor Configurations Introduction The objectives of this experiment are to examine the operating characteristics of several of the most common multi-transistor configurations, including current mirrors, current sources and sinks, and differential amplifiers. Comment The procedures in this experiment will utilize the CA3046 npn BJT array. The CA3046 is an RCA part number, and it is the same as the National Semiconductor part number LM3046. This integrated circuit comprises five npn BJTs which are fabricated on the same piece of silicon, and is a first approximation to the behavior of BJTs that one would find in a bipolar integrated circuit. The first two BJTs are tied together with a common emitter (pin 3), and the last BJT has its emitter tied to the substrate (pin 13), as shown in Fig. E3.0 below. All five npn BJTs have their collectors embedded into a common p-type substrate, which is connected to pin 13. In order to keep the collector- substrate pn-junctions reverse biased so that the BJTs will remain electrically isolated, the substrate on pin 13 MUST be tied to the lowest potential in the circuit, even if the fifth transistor is not being used. Any circuits using the fifth BJT of the CA3046 array MUST tie the emitter of this transistor to the lowest potential power supply rail. Failure to tie pin 13 to the lowest circuit potential will result in very unpredictable behavior for the circuit. Be warned!! Figure E3.0 8 Q2 Q5 Q4 11 4 3 Q1 13 9 1 2 SUBSTRATE 10 14 7 Q3 12 6 5
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Experiment-3 R. B. Darling EE-332 Laboratory Handbook Page E3.2 Procedure 1 Current mirror Comment A common need in circuit design is to establish a constant DC current for purposes of biasing a transistor, injecting a current offset, or driving a load at a constant value of current. Constant currents are established by current mirrors, current sinks, and current sources, all of which are often collectively referred to as current sources. Current mirrors replicate an existing current, current sinks pull a fixed current into a node, and current sources push a fixed current out of a node. All of these are based upon the forward active output characteristics of a BJT which provide a controllable current with a typically high value of output resistance, like an ideal current source should. The simplest description of a BJT is that the collector current is related to the base-emitter voltage as I C = I S exp(V BE /V T ), where I S and V T are constants. A current mirror is based upon the idea of keeping the base-emitter voltages of two BJTs identical, by connecting them in parallel, and thus the collector currents should also be identical. A reference current is established through one transistor, and the second transistor “mirrors” that current into an arbitrary load attached to its collector. Set-Up
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lab3 - Experiment-3 Experiment-3 Multi-Transistor...

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