### 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.
November 2008 - November 2008
© Diodes Incorporated 2008
1
www.zetex.com
www.diodes.com
AN66
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)
⎞
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.
www.zetex.com
www.diodes.com
2
Issue 1 - November 2008
© Diodes Incorporated 2008
AN66
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
Issue 1 - November 2008
© Diodes Incorporated 2008
3
www.zetex.com
www.diodes.com
AN66
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,
Figure 5 G = 10, VIN = 50mV 10kHz sine wave,
Figure 6 G = 10, VIN = 50mV 100kHz sine wave, load = 10k
www.zetex.com
www.diodes.com
4
Issue 1 - November 2008
© Diodes Incorporated 2008
AN66
Figure 7 G = 10, VIN = 50mV 1kHz square wave,
Figure 8 G = 10, VIN = 50mV 10kHz square wave,
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.
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
Issue 1 - November 2008
© Diodes Incorporated 2008
5
www.zetex.com
www.diodes.com
AN66
Definitions
Product change
Diodes Incorporated the right to alter, without notice, specifications, design, price or conditions of supply of any product or service.
Customers are solely responsible for obtaining the latest relevant information before placing orders.
Applications disclaimer
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
the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is
assumed by Diodes Inc. with respect to the accuracy or use of such information, or infringement of patents or other intellectual property
rights arising from such use or otherwise. Diodes Inc. does not assume any legal responsibility or will not be held legally liable (whether
in contract, tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business,
contract, opportunity or consequential loss in the use of these circuit applications, under any circumstances.
Life support
Diodes Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express
written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body
or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labelling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to
cause the failure of the life support device or to affect its safety or effectiveness.
Reproduction
The product specifications contained in this publication are issued to provide outline information only which (unless agreed by the
company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a
representation relating to the products or services concerned.
Terms and Conditions
All products are sold subjects to Diodes Inc. terms and conditions of sale, and this disclaimer (save in the event of a conflict between the
two when the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement.
For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Diodes’ sales office .
Quality of product
Diodes Zetex Semiconductors Limited is an ISO 9001 and TS16949 certified semiconductor manufacturer.
To ensure quality of service and products we strongly advise the purchase of parts directly from Diodes Inc. or one of our regionally
authorized distributors. For a complete listing of authorized distributors please visit: www.diodes.com
Diodes Zetex does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales channels.
ESD (Electrostatic discharge)
Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices.
The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent
of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time.
Devices suspected of being affected should be replaced.
Green compliance
Diodes Zetex Semiconductors is committed to environmental excellence in all aspects of its operations which includes meeting or
exceeding regulatory requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to reduce the use of hazardous substances and/or emissions.
All Diodes Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance
with WEEE and ELV directives.
Product status key:
“Preview”
Future device intended for production at some point. Samples may be available
“Active”
Product status recommended for new designs
“Last time buy (LTB)”
Device will be discontinued and last time buy period and delivery is in effect
“Not recommended for new designs” Device is still in production to support existing designs and production
“Obsolete”
Production has been discontinued
Datasheet status key:
“Draft version”
This term denotes a very early datasheet version and contains highly provisional information, which
may change in any manner without notice.
“Provisional version”
This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.
However, changes to the test conditions and specifications may occur, at any time and without notice.
“Issue”
This term denotes an issued datasheet containing finalized specifications. However, changes to
specifications may occur, at any time and without notice.
Sales offices
The Americas
Europe
Taiwan
Shanghai
Shenzhen
Korea
3050 E. Hillcrest Drive
Westlake Village,
CA 91362-3154
Tel: (+1) 805 446 4800
Fax: (+1) 805 446 4850
Kustermann-Park
Balanstraße 59,
D-81541 München
Germany
Tel: (+49) 894 549 490
Fax: (+49) 894 549 4949
7F, No. 50,
Hsin-Tien
Taipei, Taiwan
Tel: (+886) 289 146 000
Fax: (+886) 289 146 639
Rm. 606, No.1158
Shanghai, China
Tel: (+86) 215 241 4882
Fax (+86) 215 241 4891
ANLIAN Plaza, #4018
Futian CBD,
Shenzhen, China
Tel: (+86) 755 882 849 88
Fax: (+86) 755 882 849 99
6 Floor, Changhwa B/D,
1005-5 Yeongtong-dong,
Yeongtong-gu, Suwon-si,
Gyeonggi-do, Korea 443813
Tel: (+82) 312 731 884
Fax: (+82) 312 731 885
www.zetex.com
www.diodes.com
6
Issue 1 - November 2008
© Diodes Incorporated 2008
```