Volume 4, Issue 2, 2017
42 Available online at www.ijiere.com
International Journal of Innovative and Emerging
Research in Engineering
e-ISSN: 2394 - 3343 p-ISSN: 2394 - 5494
Analysis of High Linearity Double Balanced Gilbert Cell Down
Conversion Mixer
Jinal Shah, Nimesh Prabhakar
Department of Electronics and CommunicationLJ Institute of Engineering and Technology Ahmedabad, India
Abstract:
This paper presents a high linearity CMOS down conversion double balanced Gilbert cell mixer. In this Gilbert type mixer design, various high linearity techniques have been incorporated such as current reuse technique, differential derivative superposition method, common gate transconductance amplifier, current mirror topology, source degeneration. A comparison is made between all these techniques. Comparison shows that mixer’s Third Order Input Intercept Point and 1dB Compression point are high at current reuse technique.
Keywords: Double balanced mixer, high linearity, Gilbert cell, parameter, comparison.
I. INTRODUCTION
With the ever growing demand of wireless communications services, the development of portable communication products is rising exponentially. The super heterodyne receiver has been the most widely used architecture for modern radio communication receivers. One of the basic building blocks of super heterodyne receiver is the down conversion mixer.[1] An ideal mixer is a multiplier circuit and usually drawn with a multiplier symbol as shown in fig 1. An ideal mixer translates the modulation around one carrier frequency to another carrier frequency. A linear time invariant(LTI) circuit cannot perform frequency translation. Mixers can be realized with either time varying or non-linear circuits [2]. A mixer is a three port device consisting of LO (Local Oscillator), RF IN (Radio Frequency Input), and IF OUT (Intermediate frequency Output) port. LO port is driven by a local oscillator, which is a fixed amplitude large signal [5]. Down conversion mixer in the receiver chain translates incoming high frequency RF signal to the low frequency IF signal.
Fig.1. Ideal Mixer [2]
II. DOUBLE BALANCED GILBERT CELL MIXER
43 the LO feedthrough from the transistor M1 will be canceled by that from M3, and any feedthrough from M4 will be canceled by that from M5. Therefore, we will observe only the mixed products of RF and LO at the IF outputs. [2]
Fig.2. Double Balanced Gilbert Mixer
III .PARAMETERS OF MIXER
A. Conversion gain
Conversion Gain of the RF Mixer is depends on the input RF circuit as well the output impedance at the IF port. Conversion gain also depends on the level of LO. Conversion Gain is the ratio of the desired IF output (voltage or power) to the RF input signal value. Conversion gain is represented in dB.
gain = 20 ∗ log10
VIF
VRF
For a single balanced active mixer conversion gain is given by:
𝑔𝑎𝑖𝑛 =2 𝜋𝑔𝑚𝑅𝑑
Where 𝑔𝑚 is the transconductance of the driving stage and 𝑅𝑑 is the load at IF port.
B.1dB compression point
The IF output is directly proportional to the RF input signal amplitude. For a mixer with voltage conversion gain, the output amplitude will be equal to the input signal amplitude times the linear gain. However, just as with amplifiers, this relation does not hold true for all input levels. At some point the output power level will deviate from the ideal linear dependence on input power level. The point where the difference between the ideal linear curve and the actual output power curve is 1 dB, is referred to as the 1 dB compression point.
Volume 4, Issue 2, 2017
44 C. Third order intercept point
Due to the intermodulation effects within the mixer, third order intermodulation (𝑓𝑅𝐹± 𝑓𝐿𝑂) are generated. The
amplitude of third order intercept point is not negligible compared to the desired signal. The point at which the output level of third order intermodulation is equal to the desired output is called third order intercept point (IIP3). Generally, IIP3 point is 10 dB higher than 1dB compression point.
Fig.4. Graph of third order intercept point
IV. MIXER LINEARIZATION TECHNIQUES
In this paper, a few techniques have been proposed to improve the linearity of the mixer due to the nonlinear phenomenon of the transconductance stage and also load stage. The proposed techniques which will be discussed here are current reuse technique, differential derivative superposition method, common gate transconductance amplifier, current mirror topology, source degeneration.
A. Current reuse technique
A current reuse technique can be used to increase current through transconductance stage. Here driving current is directly given to driving MOS and hence switching MOS width can be kept to minimum. As the gate width is kept minimum for switching MOS gate-source capacitance (𝐶𝑔𝑠) reduces. Also load resistance can be increased to improve conversion gain. The
current is steered to bleeder than the switching MOS. This topology reduces the noise figure of the system. Gain of the system will increase along with the noise improvement. In current reuse, RF is given at the gate of the current source PMOS. Conversion gain for a current reuse mixer is given by:-
𝑔𝑎𝑖𝑛 = 2
𝜋(𝑔𝑚1+ 𝑔𝑚2)𝑅𝑑
Where 𝑔𝑚1 is tansconductance of driving stage and 𝑔𝑚2 is transconductance of bleeding stage. [3]
Fig.5. Current reuse technique [3] B. Differential derivative superposition method
In general, any nonlinear circuit can be expanded using Taylor series. For the case of a mixer its small-signal current can be expressed in terms only of the differential voltage at the gate, as follows:
45 Where 𝑔1𝐷𝑖𝑓𝑓 is the small-signal transconductance (1st order nonlinear coefficient) and 𝑔2𝐷𝑖𝑓𝑓 , 𝑔3𝐷𝑖𝑓𝑓 are the second and
third order nonlinear coefficients respectively. Their magnitudes define the strength of the corresponding order of nonlinearity and can be shown as follows:
𝑔1𝐷𝑖𝑓𝑓=
𝜕𝐼𝐷𝑖𝑓𝑓
𝜕𝑉𝐷𝑖𝑓𝑓
, 𝑔2𝐷𝑖𝑓𝑓=
1 2
𝜕2𝐼 𝐷𝑖𝑓𝑓
𝜕𝑉𝐷𝑖𝑓𝑓2
,
𝑔3𝐷𝑖𝑓𝑓=
1 6
𝜕3𝐼 𝐷𝑖𝑓𝑓
𝜕𝑉𝐷𝑖𝑓𝑓3
As a result, to reduce the third order inter-modulation, 𝑔3𝐷𝑖𝑓𝑓 is important and should be minimised as far as possible. In
practice, 𝑔3𝐷𝑖𝑓𝑓 has a negative peak which can be neutralised by a positive peak introduced by a differentially fed common
source pair (also called pseudo differential pair). As can be seen in Fig.6, A weak-inversion biased pseudo differential amplifier (PDA) can generate a positive peak with suitable selection of devices and bias to the pseudo differential pair, the negative peak arising from the main differential pair can be aligned with a positive peak of similar magnitude, leading to partial cancellation of 𝑔3𝐷𝑖𝑓𝑓 .Fig.6 shows such a configuration. [4]
Fig.6. Configuration of differential derivative superposition [4]
C. Common gate transconductance amplifier
One of the key point for better linearity is through maintain the Q-point stability of the MOSFET. This can be achieved by adding a source resistance at the input stage. A source resistance Rs will stabilize the Q-point against variation in transistor parameters. For example, the value of transconductance, gm varies from one transistor to another, the Q-point will not vary as much if a source resistor included in the amplifier stage. The overall voltage gain (Gv) for Common gate (C-G) stage of the mixer can be derived as:
𝐺𝑉=
𝑅𝐿 𝑅𝑖𝑛 + 𝑅𝑠
It can be observed that the gain has been more stabilized throughout the operation. [1]
Volume 4, Issue 2, 2017
46 D. Current mirror topology
The linearity of the conventional Gilbert-cell mixer is not good due to the nonlinearity of the driver stage, which becomes more serious especially at lower bias current. Because the current mirror topology is highly linear in regardless of the bias current. Therefore, the current mirrors composed of M1-M3 and M4-M6 are added into the driver stage and the degeneration resistor R1 is connected between the output sides of the two current mirror transistors, as shown in Fig.7. With proper transistor gate width of current mirror transistors M1-M3 and M4-M6, a suitable gate biasing voltage Vb1 can be obtained, and thus results in a constant transconductance over a relatively wide range of input differential voltages which in turn translates into good linearity performance for the mixer.[2]
Fig.8. Current mirror topology [2]
E. Source degeneration
The resistive source degeneration topology is presented on Fig 9. The resistance is placed at source of the differential pair transconductance stage. By increasing RS value the linearity increases but the gain decreases. The reactive source degeneration has lower NF than that with resistive degeneration. [3]
47 V. COMPARISON OF LINEARIZATION TECHNIQUE
Table 1: Comparison between techniques
Parameter A B C D E
Gain (dB) 12.5 7 -2.68 28.4 8 CP1dB
(dBm)
-10.2 -12 -20 -24.9 -11
IIP3 (dBm)
1.6 -10 -10.5 -17.7 -10
NF (dB) 11.2 10 10.5 9.62 10.4
VI. CONCLUSION
From the comparison table it can be concluded that 1 dB compression point and third order input intercept point are higher at current reuse linearization technique. So it can be employed to improve the linearity of Double Balanced Gilbert cell mixer.
REFERENCES
[1] Kumar Munusamy and Zubida Yusoff, “A high Linear CMOS Down Conversion Double Balanced Mixer”,IEEE-International Conference on Semiconductor Electronics, pp. 985-990, 2013.
[2] Ranjendra D. Kamphade and Santosh B. Patil, “A 2.4 GHz Double Balanced Differential Input Differential Output Low power High Gain Gilbert Cell Down Conversion Mixer In TSMC 180nm CMOS RF Process”, IEEE- International Conference on Electronics and Communication System, pp. 1187-1193, 2015.
[3] Hanen Thabet and Mohamed Masmoudi, “Design optimization methodology of CMOS direct Down-conversion Mixer for Wireless Sensors”, IEEE- International Conference on Design & Technology of Integrated System, pp. 1-6, 2008.
[4] Jiming Jing and David M Holburn, “Design and Analysis of A Low-Power Highly Linear Mixer”, IEEE-International Conference on Circuit theorey and Design, pp. 675-678, 2009
[5] Jacob Phil, Kare Tais and Erik Bruan, “Direct Down Conversion with Switching CMOS Mixer”, IEEE ISCAS , pp. 117-120, 2001.
[6] Shuai Huang, Xinnan Lin, Yiqun Wei, Jin He, “Derivative Superposition Method for DG MOSFET Application to RF Mixer”,IEEE -International Conference on Quality Electronics Design, pp. 361-365, 2010.
