@Keno - the basic principle of the simple diff. amplifier as shown in the first diagram (your question) is as follows:

• 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). 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) .

• In this case, the DC potential at the common emitter node must be app. Ve= 0.65...0.7 volts. Then, both npn-transistors have the necessary voltage Vbe for proper operation. Of course, there are base bias currents in to the base nodes - however, these currents can be regarded as (unwanted) secondary effects. Remember - the BJT is a voltage-driven device Ic=f(Vbe).

• 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.

• This biasing method is always used in the diff. input stages for all operational amplifiers. Sepatae biasing is necessary only if unsymmetrice power supply is used.

• As a consequence, the "beta variation" as mentioned by you (beta dependence, beta symmetry) is of less importance for proper operation of the whole circuit.

@Keno - the basic principle of the simple diff. amplifier as shown in the first diagram (your question) is as follows:

• 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). 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) .

• In this case, the DC potential at the common emitter node must be app. Ve= 0.65...0.7 volts. Then, both npn-transistors have the necessary voltage Vbe for proper operation. Of course, there are base bias currents in to the base nodes - however, these currents can be regarded as (unwanted) secondary effects. Remember - the BJT is a voltage-driven device Ic=f(Vbe).

• 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.

• This biasing method is always used in the diff. input stages for all operational amplifiers. Sepatae biasing is necessary only if unsymmetrice power supply is used.

• As a consequence, the "beta variation" as mentioned by you (beta dependence, beta symmetry) is of less importance for proper operation of the whole circuit.

• EDIT (added): Further explanation to the "automatic adjustment" mentioned above (3rd point):

For 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).

A similar effect occurs in the diff. amplifier under discussion. 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.

@Keno - the basic principle of the simple diff. amplifier as shown in the first diagram (your question) is as follows:

• 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).

• In this case, the DC potential at the common emitter node must be app. Ve= 0.65...0.7 volts. Then, both npn-transistors have the necessary voltage Vbe for proper operation. Of course, there are base bias currents in to the base nodes - however, these currents can be regarded as (unwanted) secondary effects. Remember - the BJT is a voltage-driven device Ic=f(Vbe).

• 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.

• This biasing method is always used in the diff. input stages for all operational amplifiers. Sepatae biasing is necessary only if unsymmetrice power supply is used.

• As a consequence, the "beta variation" as mentioned by you (beta dependence, beta symmetry) is of less importance for proper operation of the whole circuit.

@Keno - the basic principle of the simple diff. amplifier as shown in the first diagram (your question) is as follows:

• 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). 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) .

• In this case, the DC potential at the common emitter node must be app. Ve= 0.65...0.7 volts. Then, both npn-transistors have the necessary voltage Vbe for proper operation. Of course, there are base bias currents in to the base nodes - however, these currents can be regarded as (unwanted) secondary effects. Remember - the BJT is a voltage-driven device Ic=f(Vbe).

• 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.

• This biasing method is always used in the diff. input stages for all operational amplifiers. Sepatae biasing is necessary only if unsymmetrice power supply is used.

• As a consequence, the "beta variation" as mentioned by you (beta dependence, beta symmetry) is of less importance for proper operation of the whole circuit.