Darlington pair circuit calculations and design example
When designing a circuit using a Darlington pair, exactly the same rules are used as for designing a circuit using a standard transistor. The Darlington pair can be treated as a form of transistor with the differences of the very much higher current gain, and the higher base emitter voltage.
To illustrate how this can be done, the example of an emitter follower circuit is given below.
★ Ci를 통해 들어오는 입력임피던스 Zi에 대한 공식은
Zi = R1 || (ri + βD*Re)
(여기서 βD는 Dalington회로안의 두 트랜지스터의 전체 전류이득)
(ri : 교류등가회로로 변환했을때 발생되는 가상저항)
임피던스는 교류전류에서 발생하는 저항이다. Z=V/I
Step by step instructions:
These instructions in this Darlington pair transistor design example can only be taken as a guide because the actual circuit may differ, or the requirements for the circuit may be different.
Determine the emitter current: This is usually the starting point for the design. It can be determined from a knowledge of what the output load is.
Determine the emitter voltage: This would normally be approximately half the rail voltage as this will give the maximum voltage swing at the output.
Determine the emitter resistor: This is simply the emitter voltage divided by the emitter current. Then choose the nearest available value.
Note: These last stages all depend on each other and it may be necessary to make the calculations in a different order dependent upon what is known.
Determine the base current: This is simply the emitter current divided by the overall current gain, HFEtot
Choose the bias point for the Darlington base: This is the emitter voltage plus the overall base-emitter voltage for the Darlington (normally 1.2 to 1.4 volts).
Choose bias current for the bias potential divide: This is normally chosen to be approximately ten times the base current.
Calculate the voltage across each resistor in the bias chain: The voltage across the lower resistor is simply the base voltage. The voltage across the upper resistor is the rail voltage less the base voltage.
Calculate the resistors in the bias chain: The voltage each resistor can be calculated using the voltage in the previous step and is voltage / bias chain current. Then choose the nearest available values from the relevant resistor series.
Determine the input impedance: This is the emitter resistor times the current gain, in parallel with the lower bias chain resistor, in parallel with the upper bias chain resistor.
Determine the input capacitor value: The reactance of the input capacitor should be the same as the input impedance at the lowest frequency for a 3 dB roll off. Using the formula for the reactance of 2 pi x (Frequency, f in Hz) x (Capacitance C in farads) or 6 f C determine the value of the capacitor. Choose the next largest capacitance value available to ensure the frequency response is assured.
Calculate the output impedance: The value of the output impedance can be assumed to be low, and the impedance of the load can be assumed to dominate for most applications.
Determine the output capacitor value: The reactance of the output capacitor should be the same as the load impedance at the lowest frequency for a 3 dB roll off. Using the formula for the reactance of 2 pi x (Frequency, f in Hz) x (Capacitance C in farads) or 6 f C determine the value of the capacitor. Choose the next higher value of capacitor to ensure the frequency response is assured.
Some of the calculations are an approximation, but in view of the tolerances on the components, they give a good end result. It may be that some iteration of the calculations is required to obtain satisfactory overall results.
