AN66

A Product Line of
Diodes Incorporated
AN66
Designing with shunt regulators – AC amplifier
Peter Abiodun A. Bode, Snr. Applications Engineer, Diodes Incorporated
Introduction
A three terminal shunt regulator can be used to make a simple and effective single-supply AC
amplifier. The solution offers cost and space saving advantages. This application note presents
the details.
The amplifier
The amplifier is shown in Figure 1. The DC gain is set by R1,R2. R3 sets up reference/load current
which is its normal function. The input and output from the amplifier are necessarily AC coupled
by C1 and C2 respectively.
Vcc
C2
R3
10k
IKA
R1
100k
Vin
C1
R4
1μF
10k
R2
100k
Vout
1μF
VREF
REF1
ZR431
VK A
Figure 1 - Gain of 10 amplifier using a 3-terminal shunt regulator
The gain calculation uses the principle that the reference terminal voltage of the shunt regulator
is fixed by the feedback network and also draws negligible current. Hence a change in VIN
produces equal current change in R4 and R1.
ΔIIN =
ΔVOUT
ΔVIN
=−
R4
R1
Therefore the AC gain within the pass band, GAC, is given by,
G AC =
ΔVOUT
R1
=−
R4
ΔVIN
Design procedure
1. Set up DC conditions
a. Choose VCC from (2 ⋅ VREF + VKA(min) ) < VCC ≥ (VOUT ( pk − pk ) + VKA(min) )
b. Choose R2. A value of the order of 100k and up is recommended.
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c. Calculate R1 from
⎛ VCC − VKA (min)
⎞
R1 = R2 ⋅ ⎜⎜
− 1⎟⎟
2
⋅
V
REF
⎝
⎠
This ensures that VKA is biased at half of VCC.
d. Determine maximum load (minimum RLOAD) on the output and calculate R3 from
⎛ VCC + VKA (min)
⎞
⋅ RLOAD(min) ⎟
R3 ≤ ⎜
⎜ 2 ⋅ VOUT(pk −pk )
⎟
⎝
⎠
2. Set up AC conditions
a. Determine R4 from R4 =
R1
G AC
where GAC is the required AC gain.
R4, R1 and R2 can be scaled up or down to obtain a desired impedance gain
b. Determine the 6dB (low corner frequency cut-off) point of the amplifier, fCL, and calculate
C1 from
C1 ≥
1
2 ⋅ π ⋅ fCL ⋅ R4
c. Calculate C2 from
C2 ≥
1
2 ⋅ π ⋅ fCL ⋅ RLOAD(min)
Note that VKA(min) is usually not a quantified parameter for shunt regulators. However it is usually
less than 1.5V for most devices.
Input impedance
If the design steps above have been followed, then the input impedance, ZIN, is given by Z IN ≈ R 4 .
The user therefore has full control of the input impedance.
Output impedance
The output impedance of the amplifier, ZOUT, provided the design steps above have been
followed, is the dynamic slope resistance of the reference used and given by Z OUT ≈ R Z . RZ for most
references is typically a few hundred milliohms and therefore will not be a problem in most
applications.
Bandwidth
The amplifier behave like an operational amplifier in that it has a constant Gain-Bandwidth
Product (GBP). In a practical test carried out using the ZR431, a GBP of 1MHz was obtained. This
will vary depending on which device is used.
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Drive capability
The shunt regulator amplifier, by definition, is a Class A amplifier. This means that, even when it
is not delivering power, it is consuming 50% of the total available power. It is therefore best suited
for signal or low power applications such as driving earphones or headphones.
Nevertheless, the amplifier’s peak-to-peak current drive capability is quantified by the IKA(max)
rating of the shunt regulator. If the application demands it, this rating can be boosted by an
external transistor as shown below. Refer to AN57 for details on current-boosting a shunt
regulator.
C2
10k
R5
IKA
R1
100k
C1
R4
1μF
10k
R2
100k
Vin
ZX T P 2039F
R3
Vcc
VREF
Vout
1μF
Q1
VK A'
REF1
ZR431
GND
Figure 2 - Gain of 10 shunt regulator amplifier with current-boosted output
Stability
Some shunt regulators can become unstable when only lightly loaded. In this case it may be
necessary to preload the output with a resistor in order to maintain this minimum load
requirement. Doing this modifies RLOAD(min) and it is this modified RLOAD(min) that should be used
in the procedure when calculating R3 and C2.
Simplified circuit
If the output voltage requirement is within VREF and VKA(min) of the shunt regulator, i.e.
VOUT ( pk − pk ) ≤ (VREF − VKA(min) ) , then the simplified circuit below may be used instead. This circuit gets rid
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of R2 to give the amplifier a unity DC gain. The AC gain remains the same being determined by
R1/R4.
C2
R3
Vcc
Vout
10k
1μF
R1
100k
Vin
C1
R4
1μF
10k
VREF
REF1
ZR431
VK A
GND
GND
Figure 3 - Gain of 10 amplifier with unity DC gain
Bench Tests
The circuit in Figure 1 was built using the ZR431 shunt regulator. The following graphs show the
obtained performance. In all cases, the top trace is the input and the bottom trace output.
Figure 4 G = 10, VIN = 50mV 1kHz sine wave,
load = 10k
Figure 5 G = 10, VIN = 50mV 10kHz sine wave,
load = 200R
Figure 6 G = 10, VIN = 50mV 100kHz sine wave, load = 10k
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Figure 7 G = 10, VIN = 50mV 1kHz square wave,
load = 10k
Figure 8 G = 10, VIN = 50mV 10kHz square wave,
load = 10k
The graph below shows test result for Figure 3.
Figure 9 G = 10, VIN = 50mV 1kHz sine wave, load = 10k
Conclusion
This application note has shown that a shunt regulator can be used as an AC amplifier and that it
offers practical benefits in terms of parts rationalisation, space and cost savings.
Recommended further reading
AN67 - Designing with Shunt Regulators – mixing, adding or summing
AN57 - Designing with Shunt Regulators – Shunt Regulation
AN58 - Designing with Shunt Regulators – Series Regulation
AN59 - Designing with Shunt Regulators – Fixed Regulators and Opto-Isolation
AN60 - Designing with Shunt Regulators – Extending the operating voltage range
AN61 - Designing with Shunt Regulators – Other Applications
AN62 - Designing with Shunt Regulators – ZXRE060 Low Voltage Regulator
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The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
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