Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2000-04-20
2001-09-04
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S265000
Reexamination Certificate
active
06285256
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to complementary metal-oxide-semiconductor (CMOS) amplifiers, and more particularly to unity-gain low-power push-pull amplifiers.
BACKGROUND OF THE INVENTION
Analog amplifiers such as operational amplifiers (op amps) have been widely produced using a variety of technologies, such as Bipolar semiconductors. More recently, complementary metal-oxide-semiconductor (CMOS) technology has been employed for such analog amplifiers. See for example U.S. Pat. No. 5,670,910 by Kato, and assigned to NEC Corp. Sometimes such analog amplifiers are integrated onto the same silicon substrate as digital sub-systems, in a mixed-signal integrated circuit (IC).
FIG. 1
is a high-level diagram of an amplifier connected in a unity-gain configuration. Amplifier
10
has differential inputs + and −, known as non-inverting and inverting inputs. The difference in the voltages of signals applied to the + and − inputs is amplified and output from amplifier
10
as signal Vo. Amplifier
10
is known as a differential amplifier, since the difference in input voltages is amplified, rather than the absolute voltage on any one input.
Input voltage Vin is applied to the non-inverting + input of amplifier
10
. Both input voltage Vin and output voltage Vo are referenced to a ground, normally 0 volts. The output voltage Vo is fed back to the inverting − input of amplifier
10
. Since amplifier
10
amplifies any difference in voltage between its two inputs, amplifier
10
is in a stable steady-state condition when the two input have the same voltage. Amplifier
10
adjusts output voltage Vo until it matches the input voltage Vin. When Vo is lower than Vin, amplifier
10
sees a positive voltage difference on its inputs and drives the output voltage Vo higher until Vo reaches Vin. When Vo is above Vin, amplifier
10
sees a negative voltage difference on its inputs and drives the output voltage Vo lower until Vo reaches Vin.
Since the feedback to amplifier
10
acts to drive its output Vo to the same voltage as its input Vin, the voltage amplification or gain in steady state is one. The feedback connection of amplifier
10
is known as a unity-gain configuration. Note that the current delivered by amplifier
10
may increase if Vin increases to reduce output sink current; thus a large current gain can still be provided by the unity-gain amplifier. Such a unity-gain amplifier is sometimes known as a voltage-follower circuit. Such unity-gain amplifiers are often used as a buffer to increase drive capacity.
FIG. 2
is a circuit for a CMOS analog amplifier that implements the unity-gain amplifier of FIG.
1
. The drain of p-channel transistor
26
supplied current to resistor
28
, forming a voltage-reference generator. The IR drop through resistor
28
determines the bias voltage that is applied to the gates of p-channel transistors
26
,
22
,
20
, so that p-channel transistors
22
,
20
act as current sources with currents referenced to the current through p-channel transistor
26
.
P-channel transistors
12
,
14
form a differential pair that switch current from p-channel transistor
22
to either n-channel transistor
16
or
18
. A current mirror is set up by n-channel transistor
16
,
18
, since their gates are connected together, providing the same gate-to-source voltage Vgs. Together, p-channel transistors
22
,
12
,
14
and n-channel transistors
16
,
18
form a CMOS differential amplifier.
The drain of transistor
12
is connected to the gate of n-channel output transistor
24
. An output stage is composed of p-channel transistor
20
and n-channel transistor
24
. However, since p-channel transistor
20
has its gate driven by the bias voltage from resistor
28
, p-channel transistor
20
acts as a current source, outputting a constant current. Only the sink current through n-channel output transistor
24
varies.
The output voltage Vo is taken from the drains of transistors
20
,
24
, and fed back to the gate of p-channel transistor
14
. Thus one of the differential pair of transistors
12
,
14
is driven by Vin while the other is driven by Vo. This provides the unity-gain configuration with feedback of Vo.
When Vin rises above Vo, p-channel transistor
12
turns off more than transistor
14
, so more current passes through transistors
14
,
18
. The gate-to-source voltage of transistor
18
must rise to accommodate the higher current flow. This higher gate voltage is mirrored to n-channel transistor
16
, resulting in more current through n-channel transistor
16
. Since more current is passing through n-channel transistor
16
, but less current through p-channel transistor
12
, the drains of transistors
12
,
16
fall in voltage. This voltage drop is applied to the gate of output transistor
24
, reducing the current sink through transistor
24
. The reduced current sink, together with a constant current source from p-channel transistor
20
, raises the output voltage Vo until Vo matches the rise in Vin.
While such an analog amplifier is useful, the amount of current from the output stage is limited. The output current is equal to the current sourced from p-channel transistor
20
, minus the current sinked through n-channel output transistor
24
. Since p-channel transistor
20
has a fixed gate voltage, the source current is constant. Only the sink current changes as the gate of n-channel output transistor
24
is varied. In practical cases, the maximum source current to the output is about half of the constant current which p-channel transistor
20
can supply. The gate voltages of transistor
24
must stay within a limited range for the circuit to operate properly. This limits the variation of sink current that can occur.
Much of the current through output transistors
20
,
24
is wasted, being sent from power to ground, with a limited amount of current being sourced to the output. A large output current is often required by loading of the output, forcing high currents through transistors
20
,
24
. This high current is undesirable from a power-budget view, since the chip's power-supply current specification must increase to supply the current to p-channel transistor
24
. Heat dissipation can also be a concern, and large transistor sizes may be needed that require more die area, increasing cost.
What is desired is a unity-gain amplifier constructed from CMOS transistors. A lower-power amplifier output stage is desired that can still source and sink a large current from the output. Reduced power-to-ground current in the output stage is desired. An active push-pull output stage for a unity-gain amplifier is desired. Reduced waste current in a high-current-drive amplifier is desired.
SUMMARY OF THE INVENTION
A unity-gain amplifier has an input for receiving an input voltage and an output having an output voltage. A first differential amplifier receives the input voltage and the output voltage. It generates a first voltage on a first node. The first voltage rises when the input voltage rises above the output voltage.
An inverting stage is coupled to the first node. It receives the first voltage and generates a second voltage on a second node. The second voltage falls when the first voltage rises.
A pull-up transistor has a gate coupled to the second node. It drives a source current to the output. The pull-up transistor drives a larger source current when the second voltage is falling.
A second differential amplifier receives the input voltage and the output voltage. It generates a third voltage on a third node. The third voltage falls when the input voltage rises above the output voltage.
A pull-down transistor has a gate coupled to the third node. It drives a sink current from the output. The pull-down transistor drives a smaller source current when the second voltage is falling. The pull-up transistor drives a larger current but the pull-down transistor drives a smaller current when the input voltage rises above the output voltage. Thus the unity-gain amplifier is a push-pull differential amplifier.
In
Auvinen Stuart T.
Mottola Steven J.
Pericom Semiconductor Corp.
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