![]() The network in the common emitter path (resistor or an active circuit acting as a very large dynamic resistor) also provides heavy voltage feedback - and the result is an automatic adjustment of the voltage between B and E - nevessary for proper operation with the designes collector currents. Sepatae biasing is necessary only if unsymmetrice power supply is used.Īs a consequence, the "beta variation" as mentioned by you (beta dependence, beta symmetry) is of less importance for proper operation of the whole circuit.ĮDIT (added): Further explanation to the "automatic adjustment" mentioned above (3rd point):įor each opamp with voltage feedback an automatic adjustment takes place whch makes the input differential voltage negligible small (bcause of the huge open-loop opamp gain).Ī similar effect occurs in the diff. input stages for all operational amplifiers. This biasing method is always used in the diff. The active circuit in the common emitter path (current source Io) is able to automatically adjust the emitter potential to the value Ve required by the wanted currents Ic1=Ic2=Io/2. Remember - the BJT is a voltage-driven device Ic=f(Vbe). ![]() Of course, there are base bias currents in to the base nodes - however, these currents can be regarded as (unwanted) secondary effects. Then, both npn-transistors have the necessary voltage Vbe for proper operation. In this case, the DC potential at the common emitter node must be app. When a transistor has maximum current through it, it is said to be in a state of saturation (fully conducting). ![]() The DC base current can/must be provided by the signal source - or we can use a simple resistor to ground (signal ac coupling with a capacitor). There are two supply voltages (plus and minus) - and the whole circuit is balanced so that the dc voltage at the base nodes must be zero! This is desired because no voltage divider is required for biasing (high input resistance). amplifier as shown in the first diagram (your question) is as follows: What is actually happening here? Or can you at least show a practical example, where this kind of biasing is put into - the basic principle of the simple diff. But by forcing the Vbe to "occur" and conduct current, whereas Vbe is forced by the current source. I somehow still don't understand how a transistor could conduct any current without Vbe connected between the base and emitter of transistor. The series negative feedback (the emitter degeneration) makes the transistors act as voltage stabilizers it forces them to adjust their VBE voltages (base currents) to pass the quiescent current through their collector-emitter junctions. Analyzing the DC Voltage-divider bias circuit, we have. the differential pair is directly biased from the side of the emitters by sinking/injecting the total quiescent current. Consider the common-emitter BJT amplifier circuit shown in Figure 1. I am specially interested in the biasing of differential transistor pair (amplifier), where biasing is done only by replacing emitter resistor with active device known as constant current source, or should be done as stated in Wikipedia and many other sites: But as we use different transistors, even voltage divider cannot make amplifier beta independent. Among which voltage divider is best known for its improvements (improved temperature drift and beta variation). As mentioned with the two-supply emitter bias, these circuits are usually flipped top to bottom resulting in the flow of DC current going down the page.There are several ways to bias the quiescent point of transistor, when used as an amplifier. To create the PNP version of the voltage divider bias, we replace the NPN with a PNP and then change the sign of the power supply. Once again the proportions between voltage and current for the Q point appear to be proper when compared against the endpoints.
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