Explain the operation of a differential pair amplifier using bipolar transistors or MOSFETs.
1 Answer
A differential pair amplifier is a fundamental building block used in many electronic circuits, such as operational amplifiers (op-amps) and communication systems. It consists of two transistors (bipolar or MOSFET) with their gates or bases connected together, but their emitters or sources are biased differently. This arrangement allows the amplifier to amplify the voltage difference between the two inputs while rejecting common-mode signals.
Bipolar Transistor Differential Pair:
In a bipolar transistor differential pair, two NPN or PNP transistors are commonly used. Let's assume NPN transistors for this explanation:
Transistor Configuration: Both transistors are connected in a common-emitter configuration. The emitters of both transistors are connected to a current source (Ie) that provides bias current.
Input Stage: The inputs to the differential pair are applied to the bases of the transistors. Let's call these input signals V_in+ (positive input) and V_in- (negative input).
Biasing: The base of one transistor is connected to a positive supply voltage (V_bias), while the base of the other transistor is connected to a negative supply voltage (-V_bias). This arrangement sets up a biasing condition for the transistors.
Operation: When a differential input voltage (V_diff = V_in+ - V_in-) is applied, it creates a voltage difference between the bases of the transistors. If V_in+ > V_in-, the transistor with the base connected to V_in+ (Q1) will conduct more current, while the other transistor (Q2) will conduct less.
Current Flow: Due to the varying currents in the transistors, the current through the shared emitter resistor (Re) will change. This change in current results in a voltage difference at the output (V_out), which is taken from the connection of the two collector resistors (Rc).
Output Stage: The differential voltage at V_out is typically connected to another stage of the amplifier, like a common-emitter amplifier, to provide additional gain.
Differential Gain: The differential gain (A_diff) of the differential pair is the change in output voltage (V_out) with respect to the differential input voltage (V_diff).
Common-Mode Rejection Ratio (CMRR): Ideally, a differential pair amplifier rejects common-mode signals (when V_in+ and V_in- change together). The CMRR measures how effectively the amplifier rejects these common-mode signals.
MOSFET Differential Pair:
In a MOSFET differential pair, two MOSFETs are used. Let's consider NMOS transistors for this explanation:
Transistor Configuration: Both MOSFETs are connected in a common-source configuration. The sources of both MOSFETs are connected to a current source (Id) that provides bias current.
Input Stage: The inputs to the differential pair are applied to the gates of the MOSFETs. Let's call these input signals V_in+ (positive input) and V_in- (negative input).
Biasing: The gate of one MOSFET is connected to a positive supply voltage (V_bias), while the gate of the other MOSFET is connected to a negative supply voltage (-V_bias). This sets up a biasing condition for the MOSFETs.
Operation: When a differential input voltage (V_diff = V_in+ - V_in-) is applied, it creates a voltage difference between the gates of the MOSFETs. If V_in+ > V_in-, the MOSFET with the gate connected to V_in+ will allow more current to flow (enhancement mode), while the other MOSFET will allow less current.
Current Flow: Due to the varying currents in the MOSFETs, the current through the shared source resistor (Rs) will change. This change in current results in a voltage difference at the output (V_out), which is taken from the connection of the two drain resistors (Rd).
Output Stage: As with the bipolar differential pair, the differential voltage at V_out is typically connected to another stage of the amplifier for additional gain.
Differential Gain: The differential gain (A_diff) of the differential pair is the change in output voltage (V_out) with respect to the differential input voltage (V_diff).
Common-Mode Rejection Ratio (CMRR): The MOSFET differential pair also aims to reject common-mode signals, and the CMRR measures its ability to do so.
Overall, the differential pair amplifier using bipolar transistors or MOSFETs provides a versatile way to amplify differential signals while attenuating common-mode signals, making it a crucial component in many electronic circuits.
Bipolar Transistor Differential Pair:
In a bipolar transistor differential pair, two NPN or PNP transistors are commonly used. Let's assume NPN transistors for this explanation:
Transistor Configuration: Both transistors are connected in a common-emitter configuration. The emitters of both transistors are connected to a current source (Ie) that provides bias current.
Input Stage: The inputs to the differential pair are applied to the bases of the transistors. Let's call these input signals V_in+ (positive input) and V_in- (negative input).
Biasing: The base of one transistor is connected to a positive supply voltage (V_bias), while the base of the other transistor is connected to a negative supply voltage (-V_bias). This arrangement sets up a biasing condition for the transistors.
Operation: When a differential input voltage (V_diff = V_in+ - V_in-) is applied, it creates a voltage difference between the bases of the transistors. If V_in+ > V_in-, the transistor with the base connected to V_in+ (Q1) will conduct more current, while the other transistor (Q2) will conduct less.
Current Flow: Due to the varying currents in the transistors, the current through the shared emitter resistor (Re) will change. This change in current results in a voltage difference at the output (V_out), which is taken from the connection of the two collector resistors (Rc).
Output Stage: The differential voltage at V_out is typically connected to another stage of the amplifier, like a common-emitter amplifier, to provide additional gain.
Differential Gain: The differential gain (A_diff) of the differential pair is the change in output voltage (V_out) with respect to the differential input voltage (V_diff).
Common-Mode Rejection Ratio (CMRR): Ideally, a differential pair amplifier rejects common-mode signals (when V_in+ and V_in- change together). The CMRR measures how effectively the amplifier rejects these common-mode signals.
MOSFET Differential Pair:
In a MOSFET differential pair, two MOSFETs are used. Let's consider NMOS transistors for this explanation:
Transistor Configuration: Both MOSFETs are connected in a common-source configuration. The sources of both MOSFETs are connected to a current source (Id) that provides bias current.
Input Stage: The inputs to the differential pair are applied to the gates of the MOSFETs. Let's call these input signals V_in+ (positive input) and V_in- (negative input).
Biasing: The gate of one MOSFET is connected to a positive supply voltage (V_bias), while the gate of the other MOSFET is connected to a negative supply voltage (-V_bias). This sets up a biasing condition for the MOSFETs.
Operation: When a differential input voltage (V_diff = V_in+ - V_in-) is applied, it creates a voltage difference between the gates of the MOSFETs. If V_in+ > V_in-, the MOSFET with the gate connected to V_in+ will allow more current to flow (enhancement mode), while the other MOSFET will allow less current.
Current Flow: Due to the varying currents in the MOSFETs, the current through the shared source resistor (Rs) will change. This change in current results in a voltage difference at the output (V_out), which is taken from the connection of the two drain resistors (Rd).
Output Stage: As with the bipolar differential pair, the differential voltage at V_out is typically connected to another stage of the amplifier for additional gain.
Differential Gain: The differential gain (A_diff) of the differential pair is the change in output voltage (V_out) with respect to the differential input voltage (V_diff).
Common-Mode Rejection Ratio (CMRR): The MOSFET differential pair also aims to reject common-mode signals, and the CMRR measures its ability to do so.
Overall, the differential pair amplifier using bipolar transistors or MOSFETs provides a versatile way to amplify differential signals while attenuating common-mode signals, making it a crucial component in many electronic circuits.