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Amplifying ICs

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Overview

In this chapter you can read about:


Operational amplifier

Various operational amplifier ICs
Figure 1:
The operational amplifier is a very common device that is available in many variants as an integrated circuit on a chip. Operational amplifiers are rarely used directly for switching high currents, but are commonly used as a preamplifier stage, level converter or inverting buffer. The working principles of these circuits is treated in the chapter about operational amplifiers. This page is only about amplifying digital signals.
The photo shows ICs with two and four operational amplifiers per chip.

Operational amplifier driving an N-channel MOSFET
Figure 2:
In this circuit diagram, an operational amplifier is used as a preamplifier or level converter. The voltage divider at the input, composed of R1 and Zener diode 1, determines the "threshold voltage". An operational amplifier without feedback, as shown here, works as a comparator, which means that its output voltage is (almost) 0V whenever the voltage at the non-inverting input (labeled "+") is lower than the voltage at the inverting input (labeled "-"). The output voltage equals (almost) the supply voltage (VSupply) whenever the voltage at the non-inverting input is above that at the inverting input. These two voltages are never exactly the same in real circuits. If the difference in voltages between the two inputs is small, even the smallest variation, inevitably caused by noise, will make the output voltage jump randomly between 0V and VSupply. Since I am only treating the amplification of digital signals in this chapter, this appears only for a very short time during the transition from LOW to HIGH or vice versa from HIGH to LOW and can therefore be neglected.
An advantage is that the threshold voltage, which is determined by the value of the Zener diode Z1, can be selected to be very small, so that even the 3.3V of a Raspberry Pi are suitable for operating the amplifier circuit without any problems. If the circuit is intended to be used at only a constant supply voltage, the Zener diode can be replaced by a second ohmic resistor. With a supply voltage of 12V, the resulting threshold voltage of 1.6V can be achieved with resistors of 10kΩ (R1) and 1.5kΩ.
Zener diode number 2 and flyback diode D1 serve to protect the N-channel MOSFET against overvoltages, as described in more detail in the chapter about switching N-channel MOSFETs.

Possible components for a supply voltage ranging from 10V to 24V at up to 30A (with very good cooling of the MOSFET):
Operational amplifier: LM324N
MOSFET: z.B. IRLZ34N
Z1: 2V, 500mW
Z2: 10V, 500mW
D1: z.B. SR5100, 5A continuous current, 150A peak current
R1 and R2: 10kΩ, 0.25W

Operational amplifier driving a p-channel MOSFET
Figure 3:
In this circuit, the operational amplifier is used as an inverting preamplifier or level converter. Whenever the GPIO is on LOW (0V), the voltage at the non-inverting input ("+") is above that at the inverting input ("-"), which means that the output voltage of the operational amplifier equals (almost) the supply voltage. The P-channel MOSFET is thus switched OFF when the GPIO is also switched OFF (on LOW signal). With a HIGH signal at the GPIO, the voltage at the non-inverting input is below that at the inverting input, which means that the output of the operational amplifier equals (almost) 0V. As can be read in the chapter about switching P-channel MOSFETs, this device is switched on whenever the gate-source voltage is negative. HIGH signal on the GPIO switches the load ON.

Possible components for a supply voltage ranging from 10V to 24V at up to 19A (with very good cooling of the MOSFET):
Operational amplifier: LM324N
MOSFET: z.B. IRF9540
Z1: 2V, 500mW
Z2: 10V, 500mW
D1: z.B. SR5100, 5A continuous current, 150A peak current
R1 and R2: 10kΩ, 0.25W

Operational amplifier switching load directly
Figure 4:
Operational amplifiers can also be used to directly switch loads. However, this is a rather exotic application, which is why the maximum output currents of the most common models are rather low and are sometimes even not listed in the data sheet. The very cheap LM324N can only switch about 30mA. Currents of up to 3A at up to 60V can handle e.g. the OPA548, which is significantly more expensive than an LM324N including an N-channel MOSFET.

Possible components for a supply voltage ranging from 10V to 48V at up to 3A (with very good cooling of the operational amplifier):
Operational amplifier: OPA548
Z1: 2V, 500mW
D1: z.B. SR5100, 5A continuous current, 150A peak current
R1: 10kΩ, 0.25W

Circuit symbols of operational amplifiers
Figure 5:
This drawing shows simplified representations of an operational amplifier, which are often part of the internal layout of integrated circuits as inverter or level converter and so often appear in the drawings of data sheets.

Sink Current driver ICs

Sink Current driver ULN2803 and ULN2003
Figure 6:
If several low power loads are to be controlled, the use of driver ICs with multiple inputs and outputs is often cheaper to buy and quicker to assemble than many small signal transistors. A good example is the ULN2803A, which can switch a total of 8 channels at a supply voltage of up to 50V with up to 500mA each. The ULN2003 features 7 channels per chip.

ULN2803A channel with Darlington transistor
Figure 7:
A single channel consists of a pair of NPN transistors that are connected in the so-called Darlington configuration. Part of the internal circuit are also a 2.7kΩ series resistor that limits the base current, pull-down resistors at the inputs and a protective diode (D1) at the outputs. D2 - D4 are so-called parasitic diodes, i.e. P/N junctions within the transistors, that act like diodes.

ULN2803A internal logic and pin out
Figure 8:
The loads to be switched are connected between the positive supply voltage (VSupply) and the outputs of the chip. The (conventional) current therefore flows from the voltage source via the load into the IC. This acts as a current sink, hence the name sink current driver. The IC is designed for 5V logic voltage, whereby a current of about 2mA flows into the inputs when the HIGH level is applied. Apart from the IC, no other components are required to switch up to 8 loads.
In the equivalent circuit diagram of the chip, inverting operational amplifiers are drawn. When a LOW signal is present at the input, the supply voltage is present at the corresponding output. Since the load is connected between the positive supply voltage and the output of the operational amplifier, the positive supply voltage is present at both of these terminals and so no current flows. If HIGH is present at the input, the output of the operational amplifier is (almost) 0V and so (almost) the whole supply voltage drops between the connections of the load. Whenever HIGH signal is present at the input, the according output of the IC is at 0V, which switches the load ON.

Source Current driver ICs

Internal circuit source current driver UDN2981A
Figure 9:
The UDN2981A is an 8-channel source current driver IC. A single channel in the core also consists of a pair of NPN transistors in Darlington configuration. However, here the Darlington transistor is switched by an additional PNP transistor. Diode D1 is integrated as a flyback diode, so that the IC is suitable for switching inductive loads. Diode D2 is a "parasitic diode".

Internal logic and pin out of source current driver UDN2981A
Figure 10:
The loads to be switched are connected between ground (GND) and the outputs of the chip. The (conventional) current therefore flows out of the IC through the load to ground. The IC acts as a current source, hence the name source current driver. As with the P-channel MOSFET, the current flowing out of the chip is noted with a negative sign in the data sheet. The IC is designed for 5V logic voltage, whereby a current of less than one milliampere flows into the inputs when HIGH level is applied. Apart from the IC, no other components are required to switch up to 8 loads.
Non-inverting operational amplifiers are drawn in the equivalent circuit diagram of the chip. If a LOW signal is present at the input, 0V is also present at the corresponding output. Since the load is connected between the output of the operational amplifier and ground, there is no voltage across it's terminals and so no current flows. If HIGH is present at the input, the supply voltage is present at the output of the operational amplifier, which switches the load ON.

Sourcing parts

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If you know more ICs or common, cheap power diodes that are good for the circuits shown in this chapter, please leave a comment on this page.

Part Online shops Remark
LM324N LM324N on eBay.com

LM324N on eBay.co.uk

LM324N on Amazon com

LM324N on Amazon UK
Cheap quad operational amplifier, up to 1.3MHz
MC34074 MC34074 on eBay.com

MC34074 on eBay.co.uk

MC34074 on Amazon com

MC34074 on Amazon UK
Fast quad operational amplifier, up to 4.5MHz
ULN2003 ULN2003 on eBay.com

ULN2003 on eBay.co.uk

ULN2003 on Amazon com

ULN2003 on Amazon UK
7 channel Sink Current Driver up to 500mA per channel at 50V
ULN2803 ULN2803 on eBay.com

ULN2803 on eBay.co.uk

ULN2803 on Amazon com

ULN2803 on Amazon UK
8 channel Sink Current Driver up to 500mA per channel at 50V
TD62783 TD62783 on eBay.com

TD62783 on eBay.co.uk

TD62783 on Amazon com

TD62783 on Amazon UK
8 channel Source Current Driver up to -500mA per channel at 50V
UDN2981 UDN2981 on eBay.com

UDN2981 on eBay.co.uk

UDN2981 on Amazon com

UDN2981 on Amazon UK
8 channel Source Current Driver up to -500mA per channel at 50V
P-Channel MOSFET IRF9540 IRF9540 on eBay

IRF9540 on Amazon com

IRF9540 on Amazon UK
P-Channel MOSFET ID -23A, VDS -100V, VGS ±20V
P-Channel MOSFET IRF9Z34N IRF9Z34N on eBay

IRF9Z34N on Amazon com

IRF9Z34N on Amazon UK
P-Channel MOSFET ID -19A, VDS -55V, VGS ±20V
Schottky diode SB5100 SB5100 Schottky on eBay

SB5100 on Amazon com

SB5100 on Amazon uk
Flyback diode 5A, 100V, pulse current up to 150A
Zener diode 10V Zener diode 10V on eBay

Zener diode 10V on Amazon com

Zener diode 10V on Amazon uk
Protecting gate pin from overvoltage. 500mW are sufficient.




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