Differential Source Follower for 6 dB Gain in WiGig Filters
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Solution Overview
Problem
Designing high-performance integrated circuits at 60 GHz frequency poses challenges in achieving desired gain, bandwidth, and noise figure, particularly with reduced power supply voltage, which limits the voltage headroom and requires innovative solutions to enhance gain and frequency range in CMOS source follower circuits.
Innovation Solution
Modifying the load device in a source follower to introduce an input signal with proper phase, enhancing AC voltage gain by 6 dB, and incorporating source followers into differential amplifier and Sallen-Key filter configurations to extend high-frequency behavior and reduce the need for negative feedback networks, thereby achieving improved gain and bandwidth without increasing capacitance or die area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional source follower circuits are used with reduced power supply voltage, then power consumption is reduced, but voltage headroom and gain are limited
Solution Approach 1:
The source follower circuit is segmented into two separate devices: an active device for signal translation and a load device for impedance transformation. This segmentation allows each device to be optimized independently, enabling the load device to provide additional voltage gain while the active device maintains low power consumption.
Solution Approach 2:
The load device serves multiple functions: it provides DC bias current to the active device, transforms output impedance to a low value, and contributes AC voltage gain through its own transconductance. This multi-functionality resolves the contradiction by adding gain capability without requiring additional power supply voltage headroom.
2Stability of the object's composition
If conventional operational amplifiers are used in Sallen-Key filters, then filter stability is maintained, but high-frequency performance is limited due to bandwidth constraints
Solution Approach 1:
The invention extracts the high-frequency performance limitation from the filter design by replacing the conventional operational amplifier with a source follower configuration. The source follower's inherently wide bandwidth and low output impedance eliminate the bandwidth bottleneck while maintaining filter stability through proper impedance matching and Q-factor control.
3Reliability
If inductors are used to achieve desired filter characteristics, then filter performance is improved, but die area is significantly increased
Solution Approach 1:
The invention substitutes the mechanical inductor component with an active electronic circuit implementation using the source follower. The load device's output impedance, combined with external capacitors, creates the desired second-order filter response without requiring physical inductors, thereby achieving excellent filter performance with minimal die area.
Data Source
AI summary
Sallen-Key filters require an operational amplifier with a large input impedance and a small output impedance to meet the external filter characteristics. This invention eliminates the need for internal feedback path for stability and increases the gain of a source follower which has characteristics matching the operational amplifier in the Sallen-Key filter. The source follower provides 6 dB of AC voltage gain and is substituted for the operational amplifier in the Sallen-Key filter. The Sallen-Key filter requires a differential configuration to generate all the required signals with their complements and uses these signals in a feed forward path. Furthermore, since the source follower uses only two n-channel stacked devices, the headroom voltage is maximized to several hundred millivolts for a 1.2V voltage supply in a 40 nm CMOS technology. Thus, the required 880 MHz bandwidth of the Sallen-Key filter can be easily met using the innovative source follower.