Controlled power supply increases op amps' output-voltage range

Increasing the output voltage of IC operational amplifiers usually involves adding high-voltage external transistors. The resulting circuit then requires correction to retain its operating characteristics. This correction is difficult, especially for precise amplifiers. This Design Idea presents an alternative: the use of a controlled power supply for the operational amplifier itself, which can increase the output voltage for many precise operational amplifiers without altering their operational characteristics. You can accomplish this task by connecting controlled transistors to the power supply of the amplifier. Resistor dividers that connect to the amplifier’s output and bipolar high voltages control these transistors (Reference 1). The simple circuits in Figure 1 present general methods of connecting the amplifier as an inverter or a follower to effect increased output voltage.

Figure 1.	These simple circuits present the general methods of connecting the amplifier as an inverter or a follower to effect increased output voltage.

Dividers with resistors R₁, R₂, R₃, and R₄ determine the scale of power supply V'_PS and –V'_PS for the amplifiers. If the output voltage ranges from ±22 V, resistor R₁ = R₂ = R₃ = R₄ = R, and V_PS and –V_PS are 28 V, then voltages V'_PS and –V'_PS fall in the following range, allowing for any additional loss:

and

or

and

However, power- supply circuits include transistors, which create junction resistance, affecting the amplifier’s operation.

You can use supporting amplifiers to reduce losses and increase the quality of the output voltage of the primary amplifier. The requirements for supporting amplifiers are simple. They should have power supplies with opposite polarity from and lower applied voltage than that of the main power supply. They should provide the necessary power to the primary amplifier, and their frequency range should be slightly higher than that of the primary amplifier. You can use supporting amplifiers to eliminate the transitional resistances of transistors in power-supply connections. Thus, these circuits offer flexibility across a range of amplifier configurations (references 2 and 3).

Figure 2.	Replacing the transistors with op amps reduces losses and increases the quality of the output voltage of the primary amplifier.

Figure 2 shows an example of how to connect supporting amplifiers as followers. You derive output voltages V'_PS and –V'_PS from resistor connections with the following equations:

and

If the supporting amplifiers have a power supply of 28 V for V_PS1 and –2 V for –V_PS2 for amplifier IC₂, then –V_PS1 = –28 V, V_PS2 = 2 V for amplifier IC₃, and the output voltage of amplifier IC₁, V_OUT, is 24 V or –24 V. Also, R₆ = R₇ = R₈ = R₉ = R, such that

Further,

for V_OUTMAX = 24 V.

and

for V_OUTMIN = –24 V.

You can achieve the greatest voltage range by using separate power supplies – one for the normal voltages of the amplifier and one for the regulated part of the output voltage (Figure 3).

Figure 3.	You can achieve the greatest voltage range by using separate power supplies – one for the normal voltages of the amplifier and one for the regulated part of the output voltage.

IC₁ is the primary amplifier. Supporting amplifiers IC₂ and IC₃ have asymmetrical power supplies. You could use many types of amplifiers in this circuit, but modern operational amplifiers may be preferable because they allow the use of the complete range of the power supply and because they handle rail-to-rail input and output. In this circuit, V_PS1 = 28 V, –V_PS1 = 28 V, V_PS2 = 2 V, and –V_PS2 = 2 V.

The voltages of the primary amplifier are

Further,

Set R₆ = R₁₀ = R₁₁ = R₁₅ = R, R₇ = R₈ = R₁₂ = R₁₃ = 2R, and R₉ = R₁₄ = 3R, such that

and

Then, substitute these values into the amplifier voltages yields

and

Minimum and maximum values for each power supply are 1.5 V ⩽ V ⩽ 28.5 V, and –1.5 V ⩾ –V ⩾ –28.5 V. The total voltage of the power supply has a limit of 30 V, ranging from 1.5 to 28.5 V and from –1.5 to –28.5 V. This range permits an increase of the output voltage of the primary amplifier by ±27 V.

References