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The PNP transistor is upside down with respect the NPN transistor and thus has the correct operating voltages. If the circuit is designed so that the PNP collector voltage is a suitable value for the NPN transistor base, then there is no need for bias resistors for the NPN transistor. The circuit will amplify dc signals as well as ac ones. This is because there is no coupling capacitor to block dc. It will also amplify very low ac frequencies because there is no capacitive reactance to oppose them. However, if temperature changes cause dc drift in the first transistor, then these will be amplified by the second transistor and change its normal operating voltages.
This is usually prevented by some form of dc stabilization. |
