Phase Splitter

Phase Splitter

A phase splitter circuit produces two output signals that are equal in amplitude but opposite in phase from each other from a single input signal.

The Phase Splitter is another type of bipolar junction transistor, (BJT) configuration where a single sinusoidal input signal is split into two separate outputs that differ in phase from each other by 180 electrical degrees.

The input signal of a transistor phase splitter is applied to the base terminal with one output signal taken from the collector terminal and the second output signal taken from the emitter terminal. Thus the transistor phase splitter is a dual output amplifier producing complementary outputs from its collector and emitter terminals which are out-of-phase by 180o.

A single-transistor phase splitter circuit is nothing new as we have seen its basic building blocks in previous tutorials. the phase splitter, phase-inverter circuit combines the characteristics of a common emitter amplifier with that of a common collector amplifier. As with the CE amplifier and CC amplifier circuits, the phase splitter circuit is forward biased to operated as a linear class-A amplifier to reduce output signal distortion.

But first let’s refresh our knowledge of the common emitter (CE) amplifier circuit and the common collector (CC) amplifier circuit configurations.

Common Emitter Amplifier

common emitter amplifier

The common emitter circuit with voltage divider biasing is the most widely used linear amplifier configuration as its easy to bias and understand.

The input signal is applied to the base terminal, and the output signal is taken from across the load resistance, RL connected between the collector and the positive supply rail, VCC as shown. Thus the emitter is common to both the input and output circuits.

As well as providing voltage amplification determined by the ratio of: RL/RE, the main characteristic of the Common Emitter (CE) configuration is that it is an inverting amplifier producing a phase reversal of 180o between the input and the output signals.

To operate as a class-A amplifier the circuit is biased so that the quiescent current fed into the base, IB positions the collector terminal voltage at approximately half the supply voltage value. The ratio of resistors R1 and R2 is chosen so that the transistor is correctly biased providing maximum undistorted output signal.

Common Collector Amplifier

common collector amplifier

The common collector amplifier uses the single transistor in the common collector configuration with the collector being common to both the input and output circuits. The input signal is applied to the transistors base terminal and the output is taken from the emitter terminal as shown.

As the output signal is taken from across the emitter resistor, RE no collector resistor is used so the collector terminal is connected directly to the supply rail, VCC. This type of amplifier configuration is also known as a voltage follower or more commonly an emitter follower as the output signal follows the input signal.

The main characteristic of the Common Collector (CC) configuration is that it is a non-inverting amplifier as the input signal passes directly through the base-emitter junction to the output. Hence the output is “in-phase” with the input. Due to this it has a voltage gain of slightly less than one (unity).

As with the previous common emitter configuration, the transistor of the common collector amplifier is biased using a voltage divider network to half the supply voltage to give good stabilisation for its DC operating conditions.

Phase Splitter Configuration

If we combine the configuration of the common emitter amplifier with that of the common collector amplifier and take the outputs from both the collector and the emitter terminals at the same time, we can create a transistor circuit that produces two output signals which are equal in magnitude but inverted with respect to each other.

The Phase Splitter uses a single transistor to produce inverting and non-inverting outputs as shown.

Phase Splitter using an NPN Transistor

phase splitter circuit

We said previously that the voltage gain of the common emitter amplifier is the ratio of RL to RE, that is -RL/RE (the minus sign indicates an inverting amplifier). If we were to make these two resistors equal in value (RL = RE), then the voltage gain of the common emitter stage would be equal to -1 or unity.

As the common collector, emitter follower amplifier circuit naturally has a non-inverting voltage gain of near unity (+1), the two output signals, one from the collector and one from the emitter, will be equal in amplitude but 180o out-of-phase. This makes the unity gain transistor phase splitter circuit very useful to provide complementary or anti-phase inputs to another amplifier stage, such as a class-B push-pull power amplifier.

For proper operation, the voltage divider network connected across the supply rail and ground must be chosen to produce the correct stabilising of the DC conditions for the output voltage swing from both the collector and emitter terminals producing symmetrical outputs.

Phase Splitter Example No1

A single transistor phase splitter circuit is required to drive a push-pull power amplifier stage. Design a suitable circuit if the supply voltage is 9 volts, the Beta value of the NPN 2N3904 transistor used is 100, and the quiescent collector current is 1mA and the input signal has an amplitude of 1V peak.

To prevent distortion of the emitter terminal output signal, the d.c. biasing voltage of the emitter terminal must be greater than the maximum value of the input signal, in this case 1 volt peak. If we set the DC quiescent emitter terminal voltage at twice the input value to ensure a distortion free output swing, the VE will equal 2 volts.

As VE is set at 2 volts and the emitter current, which is also the collector quiescent current, flowing through it is given as 1mA, the value of emitter resistance, RE is calculated as:

phase splitter emitter resistance

For the voltage gain of the common emitter side of the phase splitter circuit to equal -1 (unity), the collector load resistance RL must be equal to RE. That is RL = RE = 2kΩ. Thus the voltage dropped across the collector load resistance is calculated as:

phase splitter collector voltage

Applying Kirchhoff’s Voltage Law, VCC – VC – VCE – VE = 0. Thus 9 – 2 – 5 – 2 = 0. We would expect to see this because as RL = RE and the current flowing through both resistors is approximately the same value, so the I*R voltage drop across each resistor would therefore be the same at 2.0 volts.

This means then that the DC bias voltage for the non-inverting output (emitter terminal) is 2.0 volts (0 + 2), and the DC bias voltage for the inverting output (collector terminal) is 7.0 volts (9 – 2). In other words, the DC quiescent output voltages of the two outputs are at different values.

The transistors DC current gain, Beta is given as being 100. As for a common emitter amplifier, Beta is the ratio of collector current to base current, that is; β = IC/IB, the value of the base biasing current required is calculated as:

phase splitter bias voltage

Then for a DC current gain of 100, the quiescent base current, IB(Q) is given as 10uA. It is common practice that the value of the quiescent current flowing through the base-to-ground resistor of the voltage divider network is ten times (x10) greater than the base current. Thus the current flowing through R2 will be 10*IB = 10*10uA = 100uA.

The base voltage, VB is equal to the emitter voltage VE plus the 0.7 volt forward voltage drop of the base-emitter pn-junction, that is: 2.0 + 0.7 = 2.7 volts. Therefore the value of R2 is calculated as:

resistor value

As there is 100uA flowing through R2 and 10uA flowing into the transistors base terminal, it must therefore follow that there is 110uA (100uA + 10uA) flowing through the top resistor, R1 of the voltage divider network. If the supply voltage is 9 volts and the transistors base voltage is 2.7 volts. The value of resistor R1 is calculated as:

collector-base resistor value

Thus the voltage divider network used for the DC biasing of the splitter circuit consists of R1 = 57.3kΩ and R2 = 27kΩ.

Putting the above calculated values together gives us the single transistor phase splitter circuit of:

Transistor Phase Splitter Circuit

transistor phase splitter circuit

As a single transistor phase splitter circuit produces two output versions of the input signal, a non-inverted version identical in phase to the input signal, and an 180o phase inverted version of the input signal with both outputs having a similar amplitude. This would make the phase splitter circuit ideal for use in driving push-pull or totem-pole configured outputs for amplification or DC motor control.

Consider the circuit below.

Totem-pole Output Stage

totem-pole output stage

As the complementary outputs are taken from the collector and the emitter of the transistor Q1, when the upper transistor Q2 is forward biased and conducting on the negative half-cycle (due to the inversion), the lower transistor Q3 is OFF, so the negative half of the waveform is passed to the load resistor, RL.

On the positive half-cycle of the input waveform, the lower transistor Q3 is forward biased and conducting, while the upper transistor, Q2 is OFF, so the positive half of the waveform is passed to the load resistor, RL.

Thus at any one time only one of the output transistors, Q2 or Q3 is sufficiently forward biased and conducting only one half of the input signal waveform. The two output transistors alternate conduction from one to the other as determined by Q1, with both halves of the input signal being combined together to produce an inverted output waveform across RL. Load resistor RL has a DC biasing voltage centered around the difference between VC and VE. Resistor R5 is used to limit the maximum current flow.

Transistor Phase Splitter Summary

We have seen here in this tutorial that by combining a common emitter circuit with a common collector circuit, we can create another type of single transistor circuit which is not really a CE amplifier nor a CC amplifier but instead a phase splitter circuit that produces two voltages of the same amplitude but of opposite phase.

Sometimes it is necessary to have two signals which are both equal in amplitude but are 180o out-of-phase with each other and there are different ways to create a dual output phase splitter circuit, including the use of differential amplifiers and operational amplifiers. But the single transistor phase splitter circuit configuration is the easiest to build and understand.

The single transistor phase splitter circuit is biased to operate as a class A amplifier with the two complementary (inverted and non-inverted) outputs taken from the collector and the emitter terminals respectively of the transistor. To operate correctly the gain of each output must be set to 1, unity gain.

Single transistor phase splitter circuits are useful for driving Class-B push-pull amplifiers, center-tapped transformer for inverters or totem-pole outputs for motor control, as when one transistor is ON the other transistor is OFF.

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