Buffers

What It Does

Buffers isolate signals from each other.

Use buffers to protect weaker signals from heavy loads or to prevent signals affecting each other in unwanted ways. Buffers:

• Have high input impedance, so it does not load down the signal source.
• Have low output impedance, so it can drive a high-impedance load.
• Are stable, so it does not introduce any distortion to the signal.
• Make designing circuits often much simpler.

Load example

In last section we discussed how parallel resistors or loads can affect the output voltage of a simple voltage divider. Therefore we see in the simulation below first a simple voltage divider without a buffer, and secondly with a buffer.

• Notice without the buffer the output is 4V, and with a buffers the output is perfectly 1/2 * 12V = 6V.
• The buffer prevents the load affecting the output of the divider.

Logic buffers

With transistors

The transistor buffer is simple but affective. It allows a small input signal to switch on or off a larger output signal.

• If you press the logic input from HIGH to LOW the output "copies" the logic value.
• It's a buffer: a small input current (max 5mA) controls a large output current (max 200mA).
• We have two basic versions: NPN transistors and PNP transistors.
• Notice the direction of the current of both transistors. You can also adjust the current speed visually in the simulation.
• To control an NPN transistor your control signal needs to 'source' current, while for a PNP transistor your control signal needs to 'sink' current.

Inverted logic buffer

Tri-state buffer

The tri-state buffer are logic buffers with an extra state: "HIGH-impedance". To set this third state these buffers come with an extra "enable" pin. Although they also come as dedicated IC's you probably use them already integrated with your Arduino's or other MCU's. See Charlieplexing as an example for using this buffer.

INPUT	ENABLE	OUTPUT
L	H	L
H	H	H
H	L	HIGH-imp
L	L	HIGH-imp

When the output of the tri-state buffer is:

• HIGH; it can source current (current flows out).
• LOW; it can sink current (current flows in).
• HIGH impedance; No current can flow in or out.

Non-inverting buffer

• It copies any analog voltage level to it's output 1:1 (see note below).
• The current at its input is almost zero. (we say: high input impedance).
• It can source and sink current at its output.
• No other resistors etc are needed.

Formula

Output = Input

With op-amps

The non-inverting buffer is maybe one of the easiest but yet most powerfull tools you can have.

• Op-amps don't like to receive voltages larger or more negative than they are powered at (generally). You can set it this as well in the simulator. Try out to edit the op-amp.
• A typical SDIY op-amp is the TL07x and is mostly powered with -12V and +12V at the power pins, so they are ok to receive those voltages as well.
• The opamp has a low impedance for sinking current.

With input attenuation

The non-inverting buffer with input attenuation can be used in applications where it is important to isolate the input signal from the load, but it is also necessary to attenuate the input signal. Furthermore the buffer protects the potentiometer.

Formula

Output = Input / attenuation

Warning

A potentiometer at near-zero resistance can source a lot of current which can be damaging. By using a simple op-amp buffer you can prevent this.

With transistors (emitter follower)

The emitter follower is a very simple circuit, but it is very versatile and can be used in a wide variety of applications.

• Notice a small offset between input and output. This is due to the required thresshold level to operate the transistor.
• The sinking current depends on the used resistor. This means that in practice it has a higher resistance / impedance than the opamp for sinking current.

Inverting buffer

Sometimes you want to invert your analog signal without any amplification. Use for this the inverting buffer. Make sure R1 is exactly the same as R2.

Formula (R1 = R2)

Output = -1 * Input

With op-amps

• Notice when you change the R values this circuit becomes an amplifier. See Amplifiers.

With transistors (transistor switch)

• Notice the sudden jump in voltage.

Following versus switching

Here I wanted to compare the two types of transistor buffers to make the difference extra clear.

• The emitter follower smoothly tracks the input signal, while the transistor switch abruptly amplifies the signal to its maximum voltage after crossing a small threshold around 0.6V to 0.7V.

More buffers

There are more buffers than just these, but we often don't call them "buffers", because they serve other purposes as well.

• Use "Schmitt Triggers" as logic buffers. See Schmitt Triggers.
• Use amplifiers as analog buffers. See Amplifiers.