The present invention relates to an amplifier and, more particularly, to an amplifier for controlling linear gain of wide band using external bias which controls the amplification gain at high frequencies of a wide band input signal and has good linear gain characteristics for high-frequency and large-input signal by adjusting an external bias.
In image processing systems such as video tape recorders and televisions, differential amplifiers as shown in FIG. 1A and 1B are normally employed to amplify high-frequency image signals. In these amplifiers two identical transistors Q1 and Q2 are symmetrically composed between positive and negative supply voltages V.sub.cc and -V.sub.EE and common emitter current IEE is a constant current source. Collector resistors R.sub.c 'S of the transistors Q1 and Q2 are identical with each other, and emitter resistors R.sub.e are also identical with each other. Then, an input signal Vin applied to the base of the transistor Q1 is amplified and provided through the collector resistors Rc as an output voltage V.sub.c.
On the other hand, the gain of the amplifier shown in FIG. 1A is determined as follows. Applying the Kirchhoff's voltage law to a loop including the base-emitter junctions of the transistors Q1 and Q2, the following equation is satisfied, EQU V.sub.in =V.sub.BE1 -V.sub.BE2 ( 1)
where, V.sub.BE2, and V.sub.BE2 are the base-emitter voltage drops of the transistors Q1 and Q2, respectively.
Eq.(1) can also be rewritten as follows, using the relationship I.sub.c =I.sub.s ##EQU1## where, V.sub.T (=kT/q) is the thermal voltage and has a value of about 26 mV at 300.degree. K, I.sub.s is the reverse saturation current and has a value of about 2.times.10 nA/cm.sup.2 at 300.degree. K, and I.sub.c1 and I.sub.c2 are the collector currents of the transistors Q1 and Q2.
Assuming that the transistors Q1 and Q2 are identical with each other, i.e., I.sub.s1 =I.sub.s2, then Eqs.(1) or (2) can be rewritten as, ##EQU2## Also, the following relation is satisfied, EQU I.sub.c1 =I.sub.c2 =.alpha.F.I.sub.EE ( 4)
where, .alpha.F is the current amplification ratio in common-base configuration and has a value of almost 1.
Thus, the collector currents I.sub.c1 and I.sub.c2 are given from Eqs. (3) and (4), by ##EQU3##
On the other hand, the output voltage V.sub.o1 from the transistor Q1 and the output voltage V.sub.o2 from the transistor Q2 are given by, respectively, EQU V.sub.o1 =V.sub.cc -I.sub.c1 .multidot.R.sub.c ( 7) EQU V.sub.o2 =V.sub.cc -I.sub.c2 .multidot.R.sub.c ( 8)
Then, the final differential output voltage V.sub.o becomes, ##EQU4##
As expressed in Eq.(10), when the input voltage V.sub.in is larger than V.sub.T, a large distortion is produced due to hyperbolic tangent characteristics and thereby the circuit shown in FIG. 1A is no longer used as an amplifier.
In order to compensate the distortion, resistors R.sub.e are appended to both emitters of the transistors Q1 and Q2. Then, the linearity is improved, but in has still another problem that the voltage gain is reduced.