Darlington pair

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Switching Circuits Using Bipolar Transistors

• The circuit in Figure 2 addresses the limitation of the circuit in Figure 1. 
• Two output transistors are used in this circuit, driven by a single input transistor.  
• The output of this circuit is taken from the collector of the lower transistor.  
• Just like the circuit in Figure 1, this circuit is an inverting circuit, i.e., the output signal has a phase that's opposite that of the input signal.  
• Thus, the output is low if the input is high and the output is high if the input is low.

• If the input is high, the upper output transistor goes into cut-off because its base voltage is pulled down by the conducting input transistor.
• Meanwhile, the lower output transistor saturates because the conducting input transistor is supplying its base with a higher current.  
• Such conditions immediately pulls down the collector of the lower output transistor to almost ground level, i.e., the output goes 'low'.

 

• On the other hand, if the input is low, the input transistor stops conducting, causing the voltage at the base of the upper output  transistor to be pulled up by the positive supply, thereby turning it on.
• Meanwhile, the non-conducting input transistor prevents the base of the lower output transistor from receiving any current, driving it into cut-off.
• With the lower output transistor in cut-off and the upper output transistor conducting, the output of the circuit is pulled up towards the positive supply, i.e., the output goes 'high.'

 

• The circuit in Figure 2 allows the output to switch off as fast as its switch-on, since the conducting lower output transistor immediately pulls the output to ground during switch-off.

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