Bi-polar amplifier with sharply defined amplitude limits

The invention covers an amplifier circuit in which the output is limited in either polarity. Normally, a part of the output voltage is fed back to the input to maintain linearity but when the output reaches a maximum or minimum value, any further departure is amplified and fed back to the input circuit to neutralize the part of input signals which is outside of the range to be amplified. The feedback amplifier can be designed to initiate limiting at any desired output voltages and the positive and negative output limit voltages can be asymmetric with respect to the quiescent output voltage.

This invention relates to an amplifier with an amplitude limiting circuit 
and, more particularly, to such an amplifier which can be provided as an 
integrated circuit having a strictly limited output signal. 
Amplifiers which have an output that may vary without restriction over a 
limited amplitude range but have 100 percent or more negative feedback for 
the part of any amplitude which is outside of the range have been 
available in tube type circuits for a long time and recent circuits have 
been developed using field effect transistors (FET) to do a similar 
function. Such circuits have not been satisfactory for special 
applications which require D.C. level amplification and a very abrupt 
transition from an amplifier function to a steady level output at either 
the upper or lower level limit. One such special application is the 
control of output voltage in a D.C. to D.C. voltage converter using a 
ferro-resonant transformer as shown in the article by applicant, W. B. 
Nunnery, in the IBM Technical Disclosure Bulletin, Volume 15, No. 6, Nov. 
1972, at pages 1927-1929. In this converter, a chopper between a D.C. 
power supply and the primary of a ferro-resonant transformer is controlled 
in frequency by a voltage controlled oscillator responsive to the 
rectified D.C. output from the transformer. The control voltage for the 
oscillator cannot be allowed to be less than a minimum or greater than a 
maximum limit irrespective of the transformer output, or the driving 
frequency will be such that the transformer is in an unstable region. 
Other desirable characteristics of such a control circuit are that it 
operate on a low voltage, that the circuits be designable for any 
reasonable control voltage range, that the transition between a linear 
amplification range and a fixed limit be abrupt, and that the limits are 
essentially fixed so that the limit does not vary for any input voltage 
outside of the control range. 
It is then an object of this development to provide an integrated circuit 
type of amplifier which has its output signal sharply limited to a voltage 
range by a feedback of any excess output signal to the amplifier input. 
It is another object to provide such an integrated circuit in which the 
feedback circuit includes an amplifier to limit more sharply the output 
signal to the preselected voltage limits and to do so without loading the 
input circuit. 
It is also an object of the invention to provide an amplifier circuit which 
does not use reactive circuit elements to enable use of the circuit for 
D.C. and low frequency circuits as well as the audio range A.C. 
frequencies. 
A still further object is to provide such a circuit which can be designed 
to provide any desired voltage range between the upper and lower limit 
voltages, i.e., a range which may be less than one diode drop voltage and 
may be selected without reference to diode voltage levels.

DESCRIPTION 
As pointed out above, the V.sub.out /V.sub.in relationship should, as shown 
in FIG. 1, have a linear range about a reference voltage, V.sub.REF, and 
at each end of the range should have an abrupt change to an unvarying 
output. In practice, the transition at the change points is not as sharp 
as indicated and there is always some change in the output voltage as the 
input voltage moves farther away from the transition point. These 
disadvantages are due to the requirements for some feedback signal from 
the output to compensate for input variations and can be minimized by 
making the feedback signal as effective as possible. 
FIG. 2 shows one arrangement which has been used in the prior art to 
generate the voltage relation of FIG. 1. As shown, an input signal on a 
terminal 10 is passed through a resistor 11 to the complementing input of 
an operational amplifier 12 whose output is generated on output line 13. A 
feedback resistor 15 is connected between the output line 15 and the 
complementing input of the amplifier 12. A reference voltage approximately 
midway between the end points of the operating range of the input signal 
is connected to the other input of operational amplifier 12. With these 
connections, there is a substantially linear relation between the input 
and output voltages and the amplification factor of the amplifier is 
mainly dependent on the ratio of resistors 11 and 15. 
To provide the function of limiting the output voltage, a bi-directional 
limiter circuit 20 is placed across resistor 15. This circuit senses the 
output voltage on line 13 and when the voltage is at a preset positive or 
negative voltage, will become active to pass any further changes in output 
voltage back to the input of the amplifier 12. The original circuits 20 
used zener diodes or the like to feedback the signal but this imposes a 
loading effect on the output of the amplifier, cannot hold the output at a 
fixed limit, and does not provide a satisfactory transition when it 
becomes effective. More recently, the circuit 20 has incorporated active 
components which do not load the output line 13 and provide a more 
effective feedback to hold down the output voltage changes with respect to 
input voltage variations outside the limits. 
These bi-directional limiters of the prior art are not satisfactory as the 
control circuits for the ferro-resonant power supplies described for they 
do not provide for a flat enough limited output voltage and, because of 
threshold voltage limitations, they can initiate limiting only at 
multiplies of a diode voltage drop. The embodiment shown in FIG. 3 will 
avoid these limitations of the prior art. In this figure, the 
bi-directional limiter 20 comprises an amplifier 21 having an input 
connected to the output line 13 of amplifier 12 and a dead zone amplifier 
22 having its input connected to the output of amplifier 21 and its output 
connected to both the input of amplifier 12 and to a second inverting 
input of amplifier 21. In this structure, the dead zone amplifier 22 will 
be effective to limit the input voltage of amplifier 12 as soon as the 
input voltage from amplifier 21 is high enough to exceed the dead zone 
range. This can be a substantial voltage equal to at least several diode 
voltage drops. However, this dead zone range voltage reduced by the 
amplification of amplifier 21 will be the effective voltage range at the 
output of amplifier 12 so that by proper design of amplifier 21 and 
selection of its amplifying factor, the output voltages on line 13 needed 
to initiate the limiting action can be set at any desired value. These can 
be less than one diode drop if such low limits are needed and can be 
asymmetric with respect to the reference voltage if the circuit 
application calls for such limits. 
FIG. 4 shows the schematic diagram for the preferred embodiment of the 
invention. The operational amplifier 12 shown in dotted lines at the left 
side comprises a pair of transistors 30 and 31 in a Darlington connection 
with their collectors connected to the positive lead 32 of a power supply 
through resistor 33 and the base of transistor 30 receiving the input 
signal through resistor 11. A second pair of transistors 35 and 36, also 
in a Darlington connection, have their collectors connected to lead 32 
through a resistor 37 and the base of transistor 35 is connected to a 
fixed reference voltage. The emitters of transistors 31 and 36 are 
connected together and through a common resistor 38 to the negative lead 
39 of the power supply. 
The output of transistors 30, 31, 35 and 36 is amplified by a set of four 
transistors 40, 41, 42, and 43. Transistors 40 and 41 have their emitters 
connected together and through a resistor 45 to positive line 32. The 
collector of transistor 40 is connected through a transistor 42 and a 
resistor 47 in series to the negative lead 39. Also, the collector of 
transistor 41 is connected through a transistor 43 and a resistor 48 in 
series to the lead 39. The bases of transistors 42 and 43 are connected 
together and to the collector of transistor 43 to enable these transistors 
to act as a current mirror. The bases of transistors 40 and 41 are 
connected to the collectors of transistors 36 and 31, respectively, to 
receive a push-pull pair of control signals corresponding to the input 
signal V.sub.in. The amplified output at the collector of transistor 40 is 
the amplifier output V.sub.out on line 13 and also is fed back through 
resistor 15 to the base of transistor 30. 
Amplifier 21 has a pair of transistors 51 and 52 in a Darlington connection 
with their collectors connected through a resistor 53 to positive lead 32 
and the emitter of transistor 52 is connected to one end of a resistor 54. 
The base of transistor 51 is connected to the V.sub.out line 13 to receive 
the output signal. A second pair of transistors 55 and 56, also in a 
Darlington connection, have their collectors connected to positive lead 32 
through a resistor 57 and the emitter of transistor 56 is connected to one 
end of a resistor 58. The other ends of resistors 54 and 58 are connected 
together and through a resistor 59 to the negative lead 39. The base of 
transistor 55 is connected to the input of amplifier 12. The gain of 
amplifier 21 is determined by the ratios of resistors 53:54 and 57:58 and 
is used to match the circuit design requirements to the dead zone 
characteristics. The output signal of amplifier 21 appears across two 
lines 60 and 61 connected, respectively, to the collectors of transistors 
56 and 52. 
The dead zone amplifier 22 has two identical circuits connected between 
leads 32 and 39. Each circuit comprises a resistor 62, a transistor 63, a 
transistor 64 of an opposite conductivity type, and a resistor 65 in 
series. The base of each transistor 63 is connected to the base of the 
transistor 64 in the other circuit of the pair and to one or the other of 
the collectors of transistors 51 and 55. In the connection shown, there 
will be no conduction in either circuit until the voltage difference 
between lines 60 and 61 becomes at least equal to the sum of the V.sub.BE 
drops for the two transistors 63 and 64 in series. If the voltage 
difference is such that line 61 is more positive than line 60, then the 
left pair 63, 64 will be conducting to raise the voltage of the collector 
of transistor 64 on line 67 and this will initiate conduction in a 
transistor 70 to draw current over lead 71 from the input to amplifier 12 
and thereby prevent any further voltage increase at that input. If the 
voltage V.sub.in at the input of amplifier 12 decreases to a point such 
that the voltage difference between wires 60 and 61 is sufficiently 
positive, then conduction is started in the right-hand pair of transistors 
to reduce the voltage on a wire 68 connected to the collector of the 
right-hand transistor 63. Lead 68 is connected to the base of a transistor 
69 having its emitter connected to lead 32 and its collector connected to 
lead 71. Conduction in the right pair of transistors 63, 64 will enable 
conduction in transistor 69 to feed current to lead 71 which will feed 
back to input resistor 11 to prevent any further lowering of the input 
voltage to amplifier 12. Thus, the start of conduction in a pair of 
transistors 63, 64 will feed back a current to the input circuit to 
prevent any further change in the output voltage. 
It will be seen that output voltage limiting will be initiated when the 
voltage difference between lines 60 and 61 is equal to 2V.sub.BE in either 
a positive or negative sense. The required voltage can be increased by 
inserting diodes between the emitters of a pair of transistors 63 and 64 
or the difference between the positive limit output voltage and the output 
voltage under quiescent conditions may be made different from the 
difference between the negative limited output voltage and the output 
voltage under quiescent conditions by inserting different numbers of 
diodes in the connections between the emitters of the transistors 63, 64. 
The above description of a preferred embodiment of the invention is 
illustrative only and many variations are possible within the scope of the 
invention as set out in the following claims.