Op-Amp Input And Output Current Flow Explained
Op-amps or operational amplifiers are versatile integrated circuits that can be configured to perform a wide variety of analog processing functions. A key to properly operating op-amps in circuits is to understand how current flows into and out of the device.
Input Bias Currents and Input Offset Currents
All op-amps draw a small bias current at both input terminals which is used to properly bias the internal transistor pairs. Typical input bias currents are in the range of tens or hundreds of nanoamps. The input bias currents are usually well matched, but there can be a small difference between them called the input offset current. Mismatch in the bias currents flowing into the two inputs can lead to errors and unexpected circuit behavior.
Causes of Input Currents
The bias currents originate from reverse leakage through the base-emitter junctions of the input differential pair of transistors. Some current also flows through other biasing elements. The input offset current arises from mismatches between the two input transistor paths during manufacturing.
Effects on External Circuitry
The input bias currents will flow through any impedances connected to the inputs. This can induce small unwanted voltages across those impedances that can lead to errors or unintended circuit behavior. The offset current gets multiplied by any impedances which makes balancing impedances on each input important for high precision applications.
Mitigating Input Current Issues
Use of FET input op-amps minimizes input currents significantly. For precision amplification with bipolar input op-amps, low impedance sources driving balanced impedances on each input help minimize errors from bias currents. Input bias currents impose limits on usable source impedance levels.
Output Current Limitations
The output stage of an op-amp has limits on how much current it can source or sink while keeping the output voltage within operating range. Exceeding these limits causes the output voltage to fall out of range or can even damage the op-amp. Knowing these limits helps prevent issues when designing circuits.
Dependence on Power Supply Voltages
The output currents are directly dependent on the power supply voltages to the op-amp. Higher supply voltages allow greater output currents since there is more voltage range for the output stage transistors to drop when sourcing/sinking current.
Maximum Output Current Ratings
In datasheets, parameters like maximum sourcing current and maximum sinking current specify typical limits that help determine usability for driving various loads. Short circuit and thermal overload current ratings also provide valuable information on fault conditions.
Impact on External Loads
Connecting a load drawing more current than the op-amp can deliver causes the output voltage to fall out of its proper operating range. The load only receives the current the op-amp can source/sink. Large reactive loads also stress op-amps due to charging currents.
Sourcing and Sinking Current
The output stage of an op-amp must be able both source current to a connected load as well as sink current from the load depending on the voltage potential between the amplifier output and load. This capability affects which loads can be driven.
Sourcing Current
Sourcing current refers to conventional flow of current out of the positive power supply, through the internal output transistors, into op-amp output pin, and finally into the load. This occurs when the op-amp output voltage is higher than the load.
Sinking Current
Sinking current corresponds to conventional flow of current from ground, through the load, into the op-amp’s negative output pin, then through the internal output transistors to ground. This provides a return path for current from a load at higher voltage potential than the op-amp output.
Enablement of Load Current Flow
Having internal structures capable of both sourcing and sinking current allows op-amps to properly drive resistive loads bidirectionally. With reactive loads, it allows charging and discharging capacitors or inductors through the full voltage range without getting stuck.
Example Circuit Analysis
Examining a representative op-amp circuit with specific consideration of current flow provides further insight into working with these devices.
Op-amp with Feedback Resistor and Capacitor
Here an non-inverting op-amp configuration with a feedback resistor Rf and capacitor Cf is analyzed. The input voltage Vin produces an amplified output voltage Vout applied to the parallel RC network.
Current Flow Markings on Schematic
Adding dashed arrows helps visualize the directions conventional current flows during both charging and discharging modes. Notice how the output must sink and source current bidirectionally through Rf.
Sourcing Current to Charge Capacitor
Initially when Vin is applied, Vout rapidly goes high sourcing current into Cf allowing it to charge exponentially towards Vout per the RC time constant. Rf carries this current simultaneously.
Sinking Current to Discharge Capacitor
Upon Vin getting removed, Vout drops low which reverses bias across Rf and Cf causing discharge. Cf sources current through Rf into the op-amp’s negative output terminal where it gets sunk internally, falling exponentially towards 0V.
Troubleshooting Issues Related to Current Flow
If an op-amp circuit fails unexpectedly, considering current flow offers insight into locating the fault.
Output Saturation
The output voltage sticking at the upper or lower supply rail typically means the op-amp cannot source/sink enough current for a connected load. Reducing load current demand and/or choosing an op-amp rated for higher currents may resolve this.
Thermal Shutdown
Similarly, an op-amp suddenly shutting off could signify overheating from excessive load current. The circuit may recover after cooling if no permanent damage occurred. The same resolutions apply.
Input Offset Issues
Input bias current mismatches generating unintended small input voltages can cause degraded DC precision. This may be fixed by biasing inputs carefully or selecting FET input op-amps with negligible currents.
Improving Op-Amp Current Drive Capability
When an op-amp itself lacks sufficient output current for a load, external transistors can provide additional current gain.
Emitter Follower Driver Stage
Adding an emitter follower (AKA common collector) stage using discrete bipolar transistors leverages current gain helping the previous op-amp drive increased load currents.
Buffer with Darlington Pair
Inserting an additional voltage buffer op-amp driving a Darlington pair (AKA Sziklai pair) further compounds current gain. This stacks transistor current gains enabling high load currents despite low op-amp currents.
Power Op-amp Selection
Choosing an op-amp specifically designed for driving higher power loads can avoid the need for additional stages. These contain larger internal output transistors with increased current capacity suited for loads over 500mA.
