Negative capacitance preamplifier using a dual MOS /Metal Oxide Silicon/ transistor in conjunction with bipolar transistors is used with intracellular microelectrodes in recording bioelectric potentials. Applications would include use as a pickup plate video amplifier in storage tube tests and for pH and ionization chamber measurements

Burrous, C. N.

De~boo, G. J.

English

NASA Technical Reports Server

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November 1966	 Brief 66-10549 
0-
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U. S. space 
program and to encourage their commercial application. Copies are available to the public from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Miniature Electrometer Preamplifier Effectively

Compensates for Input Capacitance 
The problem: 
To design an electrometer preamplifier that can be 
used with intracellular microelectrodes in recording 
bioelectric potentials. The requirements of such a 
device are low input current, high input impedance, 
low noise, mw output impedance, and small size. 
The solution: 
A negative capacitance preamplifier using a dual 
MOS (Metal Oxide Silicon) transistor in conjunction 
with bipolar transistors.
How it's done: 
The input signal is coupled to the gate of Qia, which 
is one half of a dual MOS transistor used in a differen-
tial amplifier configuration. Temperature compensa-
tion is achieved automatically by differential action 
and because both halves of the MOS transistor, Qi,a 
and Qib, are diffused on a single substrate. The re-
quired open loop gain is provided by Q2, Q, and 
Qs, and a low output impedance by Q6. The tem-
perature drift of Q, a constant current generator, is 
(continued overleaf) 
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compensated by CRi. Resistors Ri and R2 are im-
portant for two reasons. First, they cannot be omitted 
because the source currents then will be limited to 
the base currents of Q2 and Q . Such low currents 
will result in a voltage gain of considerably less than 
unity for Qia and Qib because of the low transcon-
ductance achieved with MOS devices at low current 
levels. Second, they can be adjusted to regulate the 
drain current so that the temperature coefficient of 
the gate-to-source voltage is approximately zero. 
Resistors R3 and R4 form a 2:1 negative-feedback 
potential divider, which sets the closed loop gain at 2. 
Resistor R5 is shunted by C  to assure stability. If the 
negative capacitance control potentiometer, R6, is 
overadjusted, the circuit will oscillate, but if the circuit 
is adjusted for critical damping, then it is stable. 
A common technique often used to reduce the 
effects of input capacitance is bootstrapping; un-
fortunately, this technique cannot compensate for 
source capacitance when it is not accessible. Using 
the amplifier in a particular application, it was de-
termined that an inaccessible input capacitance of ap-
proximately 5 picofarads existed. After adjustment 
of the negative capacitance control, an effective input 
capacitance of 0.4 picofarad was measured. This is an 
effective reduction of input capacitance by more than 
an order of magnitude.
Notes: 
1. Although MOS field effect transistors exhibit 
10-20 db greater noise levels than junction field 
effect transistors, other considerations, such as the 
acceptance of extremely low current, noncritical 
adjustment, ease of temperature compensation, 
and small size favor the use of the MOS transistor. 
2. Although the preamplifier was designed for use 
with intracellular microelectrodes, it should be 
applicable to a wide variety of measurements, 
especially where stable wideband dc amplification 
from high impedance sources is required. Such 
applications would include their use as a pickup 
plate video amplifier in storage tube tests and for 
pH and ionization chamber measurements. 
3. Inquiries concerning this invention may be di-
rected to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B66-10549 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Gordon J. DeBoo 
and Clifford N. Burrous 
(ARC-69) 
Brief 66-10549	 Category 01


Miniature electrometer preamplifier effectively compensates for input capacitance

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