MC3301, LM2900, LM3900
Biasing Circuitry
The circuitry common to all four amplifiers is shown in
Figure 11. The purpose of this circuitry is to provide biasing
voltage for the PNP and NPN current sources used in the
amplifiers.
The voltage drops across diodes CR2, CR3 and CR4 are
used as references. The voltage across resistor R1 is the
sum of the drops across CR4 and CR3 minus the VBE of Q8.
The PNP current sources (Q5, etc.) are set to the magnitude
VBE/R1 by transistor Q6. Transistor Q7 reduces base current
loading. The voltage across resistor R2 is the sum of the
voltage drops across CR2, CR3 and CR4, minus the VBE
drops of transistor Q9 and diode CR5; thus the current set is
established by CR5 in all the NPN current sources (Q10,
etc.). This technique results in current source magnitudes
which are relatively independent of the supply voltage. Q11
(Figure 7) provides circuit protection from signals that are
negative with respect to ground.
Figure 10. A Basic Operational Amplifier
VCC
Figure 11. Biasing Circuitry
VCC
Q5
Q6
10 k
Q2
(–)
Inputs
Q3
( +)
CR1
CR3
CR4
R1
3.5 k
CR5
Q1
Q4
Output
3.0 pF
CR2
Q8
R2
560
Q9
VBE/R2
Q10
VBE
R2
Q7
VBE
R1
NORMAL DESIGN PROCEDURE
1.
Output Q–Point Biasing
A. A number of techniques may be devised to bias the
quiescent output voltage to an acceptable level.
However, in terms of loop gain considerations it is
usually desirable to use the noninverting input to
effect the biasing, as shown in Figures 12 and 13.
The high impedance of the collector of the
noninverting ‘‘current mirror’’ transistor helps to
achieve the maximum loop gain for any particular
configuration. It is desirable that the noninverting
input current be in the 10
µA
to 200
µA
range.
B.
VCC Reference Voltage (see Figures 12 and 13)
The noninverting input is normally returned to the
VCC voltage (which should be well filtered) through
a resistor (Rr) allowing the input current, (Iin+) to be
within the range of 10
µA
to 200
µA.
Choosing the feedback resistor (Rf) to be equal to
1/2 Rr will now bias the amplifier output DC level to
approximately VCC/2. This allows the maximum
dynamic range of the output voltage.
C. Reference Voltage other than VCC (see Figure 14)
The biasing resistor (Rr) may be returned to a
voltage(Vr) other than VCC. By setting Rf = Rr, (still
keeping Iin +between 10
µA
and 200
µA)
the output
DC level will be equal to Vr. The expression for
determining VOdc is:
VOdc =
(Ai)(Vr)(Rf)
R
+
(
1 – f Ai
)
φ
Rr
Rr
where
φ
is the VBE drop of the input transistors
(approximately 0.6 Vdc @ +25°C and assumed
equal). Ai is the current mirror gain.
MOTOROLA ANALOG IC DEVICE DATA
2–117
ONSEMI [ ON SEMICONDUCTOR ]
