Antena Yagi

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Balun Design
In the design of mixers, push-pull amplifiers, baluns are used to link a symmetrical (balanced)
circuit to a asymmetrical (unbalanced) circuit.
Baluns are designed to have a precise 180-degree phase shift, with minimum loss and equal
balanced impedances. In power amplifiers loss of symmetry will degrade efficiency and the
symmetrical port must be well isolatedfrom ground to eliminate parasitic oscillations.
The basic construction/design of a balun consists of two 90-degree phasing lines that provide
the required 180-degree split, and this involves the use of λ/4 and λ/2.
A wire-wound transformer provides an excellent balun. Miniature wirewound transformers are
commercially available covering frequencies from low kHz to beyond 2GHz. They are oftenrealised with a centre-tapped secondary winding, if grounded this provides a short circuit to
even-mode (common-mode) signals whilst having no effect on the differential (odd-mode)
signal.
Wire-wound transformers are more expensive than the printed or lumped element baluns
described below, which find greater adoption in practical mixer designs. It should be noted
that most of these lumpedelement and printed baluns do not provide the centre-tapped
ground to even mode signals and this fact must be accounted for in the mixer design.
(1) L-C Balun
This design is essentially a bridge and is known as a ‘lattice-type’ balun. It consists of two
capacitors and two inductors, which produce the ± 90 degree phase shifts. The diagram
below (Figure 1) shows the circuit diagram of the Balun.
LRI

C
RL

Balanced
Input
C

L

Figure 1 Schematic diagram of a L-C lumped balun.

At operating frequency

ω = 2πf
L=

Zc

ω

and

; C=

Zc = R I .R L
1
ω.Zc

Unbalanced
Output

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When designing this circuit make sure that the operating frequency is well below the selfresonant frequencies of the components and take account of pad capacitances.One of the main applications of this circuit is on the output of a push-pull amplifier, which
provides a balanced signal and we want to convert to a single un-balanced output. Normally a
wound toroid style of balun is used as shown in Figure 2.
Vcc

Balun

Unbalanced
O/P

Figure 2 Use of a wire-wound balun on the output of a push-pull amplifier stage to
provide a balanced to unbalancedconversion.

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However, it may be convenient especially at chip level to realise the wound transformer type
balun, with the previously described lumped balun as shown in Figure 3.

RF
Decoupling

Vcc
L1

Bias
Choke

C1

Balanced Input

Unbalanced
O/P
L2

C2

Figure 3 The balanced to unbalanced transformation can achieved by using lumped
components instead ofa wound transformer.
(2) Transmission Line
This balun can be realised from a λ/4 length of line or coax as shown in Figure 4:
(a) 1:1 Coaxial Balun

Unbalanced
Input

Balanced
Output

Figure 4 Coaxial balun realised from a quarter length of coaxial cable, and gives a 1:1
impedance transformation.

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If we require an impedance transformation of 1:4 then we can utilisethe coaxial balun as
shown in Figure 5.
(b) 1:4 Coaxial Balun
Balanced
Output
4*R
(ohms)
Unbalanced
Input
R (ohms)

λ/2 line
Z0 = 2 * R

Figure 5 Coaxial balun realised from a quarter length of coaxial cable, and gives a 1:4
impedance transformation
(3) Microstrip
There are a wide-range of printed/micro-strip balun topologies they have the advantage of
being inexpensive,realised as they are on the Printed Circuit Board (PCB) or Microwave
Integrated Circuit (MIC) substrate. On the downside they can be quite large, particularly at
lower RF frequencies. The rat-race coupler is commonly used at microwave frequencies for
bandwidths of up to around 10-20%.
Un-balanced Input
Balanced Output

Figure 6 Simple coupled line balun
The simplest printed balun is the coupled...