Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-04-16
2003-01-21
Nguyen, Patricia (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S298000, C330S300000, C327S052000
Reexamination Certificate
active
06509797
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to integrated circuit operational amplifiers, and more particularly, to protecting inputs of an operational amplifier circuit having bipolar or junction field effect transistor inputs.
BACKGROUND OF THE RELATED TECHNOLOGY
An operational amplifier (op amp) is a high gain electronic amplifier having its gain controlled by negative feedback. Op amps are utilized in most analog electronic circuits and have become a major building block in electronic systems having sensor interfaces, low pass, high pass or band pass filters; programmable gain amplifiers, instrumentation amplifiers, input isolation amplifiers for analog-to-digital converters, and output amplifiers for digital-to-analog converters. A more thorough description of operational amplifier topologies and specifications may be found in Microchip Technology Inc., application note AN722 which is incorporated by reference herein.
The input stage for an op amp includes a pair of differentially connected transistors that receive differential input signals and provide corresponding differential currents to an active load. The inputs of bipolar transistor or junction field effect transistor (JFET) op amps are connected to a transistor junction, rather than to an insulated gate as in metal oxide semiconductor (MOS) transistor technology. The bipolar or JFET type op amps can exhibit an undesirable characteristic known as phase reversal should either of the transistor junctions associated with the op amp inputs become forward biased. The transistor junctions become forward biased if the inputs thereto are above or below the voltage supply rails. If this condition should occur, a large reverse current flows (if a bipolar junction) and causes the op amp output to switch to an incorrect state.
The phase of the input signals is a relative term and is defined by the polarity of one signal with respect to the other signal. An op amp is in phase when the phases of the input signals and differential currents of the op amp are the same. Phase reversal of an op amp occurs when the phase of the differential currents is opposite the phase of the input signals. The amplifier operates over a Common Mode Range (CMR) of input signals that lie between the high and low supply voltages. In the case of a single-voltage to ground supply, the CMR includes one of the supply voltages, typically the low supply which is commonly ground reference potential. If one of the input signals is driven outside of the CMR due to noise or improper drive circuitry, the corresponding differential transistor will either turn off or form a forward biased parasitic diode. When the transistor junction forward biases, the phase of the differential currents reverses with respect to the phase of the input signals and may cause the op amp to malfunction and latch up (stop working).
Most bipolar and JET op amps have additional circuitry to protect against phase reversal. A pair of phase compensation diodes may be cross-couple connected between the differential inputs and the other differential transistors' collectors to prevent phase reversal of the amplifier. When either one of the differential transistors' collector-base junctions is forward biased, the corresponding cross-coupled diode conducts and prevents a phase reversal of the differential currents. However, this type of phase reversal prevention greatly increases the magnitudes of the differential currents, on the order of twenty to thirty times the normal current values. Although op amp failure is less likely to occur, these extremely high current levels can still cause the active load to malfunction and latch up the op amp.
These phase reversal protection diodes at the inputs of the op amp are supposed to conduct before the input diode junctions of the differential input transistors of the op amp, thus clamping the signal paths to maintain the correct output state. Therefore, the input diode junctions cannot be forward biased, thus no phase reversal. This solution is limited in that the clamping diodes must have a forward voltage drop of less than the input diode junctions of the differential input transistors of the op amp. A diode's forward voltage is proportional to the natural log of the diode area, so the protection diodes may be quite large in area on the silicone substrate because an allowance of some margin for a smaller diode forward voltage than the input differential pair must be used. This requirement for the protection diodes forward voltage drop and increased area requirements may require additional processing steps in the fabrication of the op amp, or changes to the input differential transistor pair of the op amp.
It may also limit the amount of forward current that the op amp input can handle before phase reversal may occur. Conduction or turn on of the protection diodes is relatively gradual and varies significantly with temperature, thus making the clamping threshold of these diodes imprecise.
Referring to
FIG. 1
, a prior art schematic diagram of an operational amplifier input stage having diode phase reversal protection is illustrated. A typical single power supply (ground and a positive voltage) input stage of a bipolar transistor integrated circuit operational amplifier (op amp) is generally indicated by the numeral
100
. One skilled in the art of analog integrated circuits would also recognize replacement of the bipolar transistors with junction field effect transistors, also contemplated herein and within the scope of the present invention.
The op amp input stage
100
includes cross-coupled phase compensation diodes for preventing phase reversal, and current compensation diodes for preventing overshoot of the differential currents. A pair of differentially connected transistors
102
and
104
, which can be either bipolar or junction field effect transistors (JFETs), have their current circuits connected together on one side to divide the output of a current source
106
. As used herein, a transistor's “current circuit” refers to the collector-emitter circuit of a bipolar transistor, or the source-drain circuit of a JFET; a transistor's “control circuit” refers to the base of a bipolar device, or the gate of an JFET. In the circuit illustrated in
FIG. 1
, the differential transistors are bipolar pnp transistors.
Input terminals
108
and
110
are adapted to receive differential input signals through series resistors
112
and
114
. In the manner characteristic of differential amplifiers, the transistors
102
and
104
divide the current from the current source
106
in mutual opposition, with the amount of current through each of the transistors
102
and
104
varying according to the relative input voltage signals applied to their bases from the input terminals
108
and
110
, respectively. If a constant, known bias is applied to one of the input transistor bases, the magnitude of the signal at the base of the other input transistor can be determined by the amount of current flowing through that transistor.
The input current source
106
operates from a positive voltage supply, V
CC
, while the collectors of the pnp input transistors
102
and
104
are connected to a negative voltage supply V− (preferably ground potential) through respective series-connected first and second trimmable input resistors
116
and
118
. The trimmable resistors
116
and
118
may be used to minimize any offset voltage of the input circuit.
The input stage includes a folded cascode pair of active load npn bipolar transistors
120
and
122
, whose emitters are connected to the collectors of input transistors
104
and
102
, respectively. The bases of transistors
120
and
122
are connected together for common biasing. A bias circuit for the transistors
120
and
122
consists of a current source
124
that is connected to the bases of the transistors
120
and
122
and to a diode
126
, which is connected through resistor
128
to ground potential. Diode
128
can be a diode-connected transistor. The magnitudes and
Baker & Botts L.L.P.
Microchip Technology Incorporated
Nguyen Patricia
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