  The readout of transition-edge sensor (TES) bolometers with a large
multiplexing factor is key for the next generation Cosmic Microwave Background
(CMB) experiment, Polarbear-2, having 7,588 TES bolometers. To enable the large
arrays, we have been developing a readout system with a multiplexing factor of
40 in the frequency domain. Extending that architecture to 40 bolometers
requires an increase in the bandwidth of the SQUID electronics above 4 MHz.
This paper focuses on cryogenic readout and shows how it affects cross talk and
the responsivity of the TES bolometers. A series resistance, such as equivalent
series resistance (ESR) of capacitors for LC filters, leads to non-linear
response of the bolometers. A wiring inductance modulates a voltage across the
bolometers and causes cross talk. They should be controlled well to reduce
systematic errors in CMB observations. We have been developing a cryogenic
readout with a low series impedance and have tuned bolometers in the middle of
their transition at a high frequency (> 3 MHz).
