Many times we need to design DC level shifting circuit to shift voltage level from one to another such as from +5V to 0V as is done in Operational Amplifier with minimum change in the ac voltage signal that rides on such dc signal. Here are some of the methods one can employ.
Emitter Follower Circuit
One simple way to do this is to use transistor in emitter follower(emitter follower is called so because its output voltage at the emitter "follows" the input voltage applied to the base with only a small voltage drop, typically equal to the base-emitter voltage () of around 0.6–0.7V for silicon BJTs) circuit configuration shown below to reduce the input voltage(\(V_{in}\)) by the fixed base to emitter voltage(\(V_{BE}\)) to give reduced voltage output.
\[V_{out}= V_{in} -V_{BE}\]
In the circuit diagram the output voltage is 2.28V reduced from 3V by 0.72V.
Common Collector Stage Circuit
A shift of just 0.72V is generally inadequate. So we can change the above circuit to include a voltage divider with two resistors \(R_1\) and \(R_2\). This circuit is shown below and the circuit configuration is called common collector stage. This circuit also acts as a buffer to isolate high gain stages from the output stage when used to construct operational amplifier.
The output voltage can be found out by applying KVL from input to the ground:
\[ V_{in} = V_{BE} + I(R_1+R_2)\]
so, \[ I = \frac{V_{in} - V_{BE}}{(R_1+R_2)} \]
Since, \[ V_{out} = IR_2 \]
Hence, \[ V_{out} = \frac{(V_{in} - V_{BE})R_2}{(R_1+R_2)} \]
Simplify the terms:
The main drawback of using this dc level shifting circuit is the ac signal voltage is also attenuated by the factor \( \frac{R_2}{(R_1+R_2)}\). As \(R_2\) decreases to improve the dc voltage level the ac gain decreases. Another disadvantage of this circuit is that its output impedance is high.
Constant Current Source Circuit
The improvement to the above circuits is to use constant current bias setup circuit as shown below.
The expression for the output voltage for this circuit is as follows,
\[ V_{out} = V_{in} - V_{BE} - I_1 R_1 \]
Here the output voltage can be adjusted with current \(I_1\) and/or resistor \(R_1\) as illustrated in the animated circuit diagram above(RV1 which is a pot is used as \(R_1\)). See the role of constant current bias in BJT differential amplifier circuit.
Current Mirror Circuit
Another technique to produce dc voltage level shifting is to use current mirror circuit as shown below.
In current mirror circuit, the current flowing through resistor \(R_1\) (here represented by RV1(POT) to demonstrate) and the current flowing through resistor \(R_1\) into the base of Q2 are equal.
That is, \(I_1 = I\)
Since, \[ V_{EE} = I R_2 + V_{BE}\]
\[I_1 = I = \frac{V_{EE} - V_{BE}}{R_2}\]
And, \[ V_{out} = V_{in} - V_{BE} - I_1 R_1 \]
Hence the output voltage can be controlled with proper setting of current \(I_1\) and/or resistor \(R_1\) as illustrated in the animated
circuit diagram above(RV1 which is a pot is used as \(R_1\)). To learn more about this circuit see Current Mirror working principle and application.
\(V_{BE}\) Multiplier Circuit
Another method of leel shifting network is the \(V_{BE}\) multiplier circuit which is shown below.
If the current flowing through Q1 emitter is I, then neglecting the current flowing into the base of Q2, the same current will flow throught the resistor \(R_2\).
So we can write, \[V_{AB}=I(R_1+R_2)\]
Applying KVL to base emitter loop of Q2 gives,
\[I R_2 = V_{BE}\]
and \[I=\frac{V_{BE}}{R_2}\]
and therefore, , \[V_{AB}=\frac{V_{BE}(R_1+R_2)}{R_2}=V_{BE}(1+\frac{R_1}{R_2})\]
Hence this circuit is called \(V_{BE}\) multiplier circuit.
The voltage output is given by,
\[V_{out}= V_{in} - V_{BE}- V_{AB}\]
\[V_{out}= V_{in} - [V_{BE}- V_{BE}(1+\frac{R_1}{R_2})]\]
Thus,
\[V_{out}= V_{in} - V_{BE}[(1+\frac{R_1}{R_2})]\]
From the voltage output equation we can see that the output voltage can be set adjusting the value of \(R_1\) and \(R_2\). This is also illustrated in the animated \(V_{BE}\) multiplier circuit. This type of circuit is used in 741 operational amplifier circuit.
Lateral PNP & Vertical NPN Transistor Circuit
Yet another method to change the dc voltage level in electronics circuit is to use Lateral PNP & Vertical NPN Transistor combined circuit as shown below.
Here for such circuit the output voltage is given by the following equation.
\[V_{out}= \frac{R_1}{R_2}(V_{CC} - V_{BE})\]
