Current Sources b

# Current Sources b - Current Source Biasing Integrated...

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Current Source Biasing Integrated circuits have transistors which are manufactured simultaneously with the same device parameters (parameters from chip to chip will vary) As a result, different bias techniques are employed than in discrete designs One common technique is current source biasing, which allows the designer to take advantage of matched devices We will begin by looking at some simple current source circuits A current source is not a “naturally” occurring device. It can be simulated by a network of transistors and circuit elements.

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The voltage across R E is approximately constant. I E is held at a constant value I V V R E EE BE E = - -

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Problem: For the previous circuits find the bias values I C and V CE for each transistor Solution Assume D 1 and D 2 forward biases (I 1 > I B2 ) V V V V V I R V V I V R I I I I B EE F F BE E BE F E F C E C C = + = - + = - = + 2245 = = 2245 2245 = = 2 10 2 0 7 8 6 0 7 180 37 39 2 2 2 2 2 2 1 2 V V) V Using KVL around loop A 2 Since V mA Since mA ( . . . . .

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Check that D 1 and D 2 are forward biased for a worst case minimum β F = 20 I I I V V R V V I R V V V V V V V V V V V V B C F CC B C CC C C E B BE E C BE CE C E CE C E 2 2 1 1 1 1 2 2 1 2 1 1 1 1 2 2 2 019 10 8 6 50 037 10 39 61 8 6 0 7 93 0 07 0 7 68 0 7 8 6 = 2245 = - = - - = = - = - = = - = - - = - = = - = - = - = - - = = - = - - - = b . ( . ) . ( . ) . . . . . . ( . . . ( . . mA V V k mA V mA)(1k V V V V V V V V) V V V) V
Current Mirrors Current mirrors also take advantage of matched transistors but require a minimal number of resistors. They are also well suited for circuits with more than one stage.

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Basic BJT Current Mirror

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I V V V R V V V R I I I I I I I I I I A CC CE EE A CC BE EE A A REF B B B REF B REF REF A = - + = - - = + + << << 2245 ( ) 1 1 1 2 1 2 IF is large F b Problem For the following circuit I C3 = 3 mA and V CE = 5.4 V. Find the quiescent (DC bias) power dissipated in each transistor.

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I I I I I V V R R V V I V V V V V R C o REF A A F EE A A F EE REF E CE C CC C C 3 0 07 10 31 0 7 54 47 10 4 7 53 3 18 2245 = 2245 = - + = - - = - - - = = - = = - = - = = = V V V) 3mA k V; To achieve V V V V V V mA k ( ) . ( . . . , . . . . .
The DC power in each transistor is given by: P I V I V I V P P P Q C CE B BE C CE Q Q Q = + 2245 = = = - - - 2245 = = 1 2 3 3 0 7 0 2 3 0 7 10 28 3 16 ( ( . .

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Current Sources b - Current Source Biasing Integrated...

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